Just my 2 cents worth...
I think the first question is whether to buy new or used...
Personally I don't mind buying used lenses, but prefer to get a new camera body with a warranty. There's a lot more to go wrong in a camera body than in a lens. You can look at a lens and tell if the optics are clean and in good condition, and testing it's autofocus and aperture control functions are simple enough.
With a new camera body, if it fails due to a flaw or a weak component it will usually do so within the first few weeks, months or up to a year of use (depending upon individual rate of usage... In my case it's probably weeks ). If it survives the initial warranty period, a camera body will quite likely have a long, happy life... At least up to the time the need to upgrade to the latest and greatest model kicks in.
That being said, the question might then come down to a new Canon 50D with a kit lens vs a used D300 with or without a lens. Personally I'd go with the new Canon.
The D300 is a fine camera, but at a higher price. I don't feel that the Nikon AF system is all that superior... I've had and used the Canon 45 point AF system for years, and simply have never used most of it... Nine to 11 points are plenty, since I use only the center one 80 to 90% of the time, along with the separted focus technique (CFn 4 and the * button on older models, two different CFn and either the * or AF On button on more recent Canon).
As far as I'm concerned, it's the placement of the AF points in the image area that matters, not the sheer number of them. In fact I always used my 45-point Canon AF "dialed down" to 13 points (via a custom function), albeit with the spot metering linked to the active point (a useful feature not now possible with anything other than the 1-series Canons... I don't know about the Nikon models). I really don't need 51 points, nor do I consider it "superior" simply due to a large number of points crowded into the center of the image area.
With sheer number of AF points aside, we're left comparing AF speed, accuracy, lock on and ability in lower light situations. I use three 50D and find it quite responsive to most conditions, so long as reasonbly fast and quick focusing lenses are used. You could probably look up the tests of the D300 AF for comparison, I'm not experieinced enough with it to offer a comparison. I can tell you for certain 5D Mark II's AF is noticeably slower, but accurate especially with the AF assist points enabled. I prefer the 50D for moving subjects. I'm sure the current 1D series would be even better with moving targets, but of course at a considerably higher price.
I think you'd be best advised to put more of your budget into lenses, rather than into the camera body. For one, better quality lenses will make for better images. Most modern camera bodies are relatively equally capable of capturing great images, when used correctly. Among camera bodies, the differences are more in the features and the ergonomics. With lenses the differences ultimately show up in image quality.
Plus, your lenses will likely still serve you with the next camera and the one after that, etc. Some of my lenses that were first purchased in 2001 have been used on at least a dozen different cameras - first film and now digital - in the years since.
If your budget is $3000 US, you can pick up a 50D in a kit with 28-135 for less than half of that. Get the hood for that lens, though. It needs it.
Need a wider lens? The Canon 10-22 will set you back about $700 or so (and the hood is a separate item). The Tokina 12-24 is around $500 and a good alternative, in my experience.
A 430EX or 580EX might run you another $300-500, with an off camera cord and bracket.
You don't need "protection" filters. They're a waste of money when on a tight budget. However a circular polarizer is a very useful filter. A good one in 72mm or 77mm size will cost about $150. (I tend to use polarizers a lot more on wide angle lenses, but that's just me.)
Shop around carefully and you'll probably still have money left for a camera bag, a decent tripod, a spare battery or two (Sterlingtek are half the price of Canon... eBay generics are 1/4 to 1/6 the price of Canon), as well as some memory cards. Might even have the money for a used BG-E2 or BG-E2N if you want one.
Then start saving for additional lenses to add eventually, if and when you need them. Personally I'd want a 50/1.4 (either the Canon or the Sigma) and/or an 85/1.8 (I have heard rumor of a Sigma 85/1.4 coming out... sounds interesting). Perhaps a 60mm to 100mm macro, but the 28-135 does macro pretty well, and can be used with extension tubes ($160 for a Kenko set) for even higher magnification.
Personally I use longer prime lenses a lot, too. I just added a used 300/4 IS to my kit. A lot of people really like the 200/2.8 II, I just wish it had I.S.
Speaking of which, I really like I.S. and have been using some lenses with it for many years. However, I feel it's most useful on 70mm, 100mm and longer lenses. I just don't find it very necessary on shorter focal lengths, especially as cameras become more and more capable of high ISOs. It's not very helpful on macro lenses, either. But, if a lens happens to have it, so long image quality isn't negatively effected by the additional optical elements needed by it and the lens isn't higher priced as a result, what the heck... I'd certainly take I.S. on any lens.
In a later post you suggest you might be offering photo services of some sort. Well, that brings up another thing that will effect your budget...
If you are shooting for pay and taking on jobs, you really shouldn't have fewer than two camera bodies. You might be able to finish the job you have commited to do if a lens breaks or a flash craps out... But if you only have one camera and it breaks, you are out of business and going to have a very upset client. If you plan to take on work, you might be better looking for a pair of used XTi's or something equivalent.