If you already have a decent PC, jsut spend all the $$ on a gfx card. The wiki isnt entirely accurate, but its a really good estimate. Typically if you are not getting similar results its a config issue.
The wiki is off and/or not exactly, but with my 2 HD 6970, I'm get 500 kh/s each. That's pretty close to what the wiki has them rated and I got them both for $220 on Craigslist. I'm mining on a budget myself and also got 2 HD 5870's on another setup. Pulling in total, 1600~1800 kh/s.