Lol... ok here I go again... I'm gonna pimp my budget setup...
Biostar G6100 mobo s 754
Sempron64 (the number you get will depend on your budget)
Gig of RAM, bout ~$70.
Now That's low ending it. I'd shoot for a s939...like this setup...
You can get that for about $170, but if you have $500 then by all means try for one of these setups....
Having the 9250 is pointless, I have one and NVidia's new G6100 integrated chipset runs better than the thing.
With that budget you could get something like...
Here's a possible dual core setup, you'd get the processor, a G6100 mobo and 1gb ram. Use the onboard video till you save up for a 6600GT or 6800GS.
That's the avalible ones, it's on the G6150 which is a little better than the std G6100, the Gigabyte, ASUS, and MSI are all good brands so it's up to you.
OCZ 1gb RAM
Tho I'd also suggest you pick up a really cheap stick of 128 or 256mb ram because you'll need to allocate it to the onboard video.
Or, if you feel like sticking to a single core proc with better gfx now...(which I'd say nab a dualcore and wait to get an awsome gfx card)
Here's a possible set up.
This is fine for quite a bit of stuff, you could always OC it if you had to
OCZ 1gb RAM
Same ram, it should be pretty good
Then get a 6800GS. I'd buy the eVGA or Gigabyte one, it's up to you.
With a $500 budget there's no way you shouldn't jump to a new socket and have pcie as an option. I'd go for the setup with the dualcore proccessor, that'd be the best for future proofing your comp, and then you could buy something like the 6800GS later. I'd say go with that because the onboard GFX for that mobo performs as well, or better than the GFX card you have now, and is way more compatible with games. But if you want much better GFX now then go w/ the 2nd setup, but that one goes over by about $25.
Hope that help you get an idea of how much you can steach $500 out to. I see way to many people spend money only to have to spend a ton more just a little farther down the road.