If money was less of an object I'd have gone full frame long ago and I would be running to get a 5D II and a 50D - one for telephoto and fast burst and the other for wider angle and full frame narrow DoF. That would be a very good setup for me.
I have a 20D and a 40D and the ISO difference is not significant between the two, but the AF system on the 40D is much better in low light. The main draws of the 50D have not made me feel compelled to upgrade; the price is too great and I don't believe the new higher ISO settings offer improved RAW image quality over pushed 40D files. I also no longer have a strong desire to increase my megapixels because my computer already can't handle the 10mp files, and honestly the resolution isn't going to make a huge difference to me most of the time anymore. If I had a 5D II I'd probably use it mostly at sRAW, 10 mp. I want it for the ISO capability and DoF.
After this month's wedding marathon, the original desires still stand: The two things I want most are high ISO settings with low noise, and full frame's narrow DoF. There are many, many shots I can't produce the way I want to with the lenses I have because they're too long on my system and the aperture isn't fast enough to blur the background adequately. Using a 24-70mm f/2.8 lens on a crop sensor camera guarantees that full and half body shots on the wide end will have very little background blur, but I get the impression that full frame would produce that blur for me much better. And, if I could combine a fast wide prime and a full frame camera, I would surely be able to get a type of shot that I can't possibly create with my current cameras.
So anyway...I believe - not from experience - that full frame would give a noticeable change in the style of your photos. However, I also believe that the 40D's AF system is a big help in many situations over the 20D. Live view is also extremely useful for detail shots. What's more, you can probably get two slightly used 40Ds for the cost of one new 50D - to me, the 40Ds would be a better deal, but if you want the higher ISO "settings" of the 50D (instead of just pushing files), or if you want the higher resolution or other features...well okay, get the 50D.
If it were me with two 20Ds I'd evaluate my finances: If I could afford it, the 5D II would be my first choice for a wedding camera. If not, the 40D and 50D both make good options, but the 40D probably offers more for the money overall, though not significant improvement in the ISO sector - though I don't think the 50D offers better ISO either, just higher available settings. I have thought about the 5D several times and almost bought one...I suppose if it had a few more features (live view, ISO6400) and faster response and AF I'd have already bought one...but I haven't been able to justify it. I guess that I'll buy a 5D II when it comes down to about $1200...