Actually the 5700 is faster then the 4200 in newer games. Slower in older titles. Anyway, Jake, you could get the 5700LE that I have. It comes out of the box overclocked and can be pushed further to full 5700 speeds. However, you might want to skip both of those and go for the 6200 AGP. I'm getting the leadtek one from Newegg and it's only $124 shipped and from what I've seen in my researh it performs between a 5700U and 5900. And if you'er lucky enough to be able to unlock the 4 additional pipes it's faster then the 5900 in most cases.
Check the VGA Chart
here at Toms Hardware. You'll find that the 5700 does quite well vs. the 4200.