What to buy-Rebel XSi or 430EX II flash+Canon 24mm f/2.8?

I'm currently using my brothers rebel XT since he doesn't use it anymore. I bought a nifty fifty lens. He has a speedlite but it's a cheap offbrand one that doesn't let me control anything other than switching it from regular flash to VERY strong flash. He also has the kit lens but I hate it so it never gets used, but I do want that wide angle. I can pretty much buy either or right now, just wondering whats best. A newer upgraded camera with the kit lens (is it really that much better than the XT kit lens? Will I be able to get away with this lens quality for at least another year?) Or should I get the wide angle lens and the flash. I'd REALLY love the flash for the ttl purpose and the red af assist (the camera doesn't read the flash really so when I take night pics with no light it's virtually impossible to get a focus) And the wide angle would be amazing to have since I've been only using the nifty fifty. Any advice please! And before anyone reccomends pricier things I can't spend more than about 500-600.

Well, the flash and new lens will work on the camera you have now (unless your brother changes his mind); if you get the XSi you'll have a lens that's not too different from the XT kit (18-55) lens aside from the IS. You can always upgrade the body later; the lens and flash will last a lot longer than the body will and they'll expand your capabilities right now. The main thing an XSi will get you is more MP than the XT has.

Thats what I was thinking, being that I'm hoping to be able to buy one or the other in the next year or 2 so if that's the case I'd wait to buy the t2i anyhow. I put an order in at adorama and something came back wrong so they said to call them but NOW it's a little under an hour too late to get it shipped same day and because of a holiday coming up they wont ship till friday so I'll be waiting to order till tomrrow to think it over more. Still wondering if the lens/flash purchase will help me or not that much. I mean I know it will improve and such just wondering if it's worth it right now.

My philosophy has been (and remains) "get it if you're bumping into limits because you don't have it". If you're being limited by what the XT can't do, but the XSi, or the T2i, can then you're ready for a new camera. If, on the other hand, the pop-up flash isn't strong enough, or the kit lens is too slow too often, the flash and/or the 24 will help you more than a new camera body.

I'm not bumping into things camerawise yet but his flash I can only really use pointed up bounced and such so if I need just a tad of fill light for outdoor shoots (which I'm trying to get into right now) I can't do it or I have to cover the flash some and it's still using all the flash power kwim? Lens wise I know I could use his kit lens but the sharpness of a prime is what i crave esp after the nifty fifty. His kit lens isn't very good and he got it used so idk if thats the norm or just his lens. I think I've decided on the lens/flash but still got a couple days to think it over.

flash will change your photos BIG TIME.
I was in a toss up like this.. 430ex and 70-200f/4 VS Canon 50D body.
Went with the flash and lens. I love it.

i say go for the t1i or the 40d instead of the xsi if you can.
If not go for the flash and the 24 f/2.8 because you will most likely want a flash for the future.

Canon currently has a rebate on the 430 ex II so now might be the time to bite.