After reading these comments related to the Dell C521. I own a Dell C521 & agree the on-board graphics are yuck!
Aren't ALL on-board graphics yuck? Fine if you just need for work. But as far are the Leadtek 7600Gs being your best bet, I disagree. It is true that this machine is housed in a slim BTX case, with a small power supply, which limits the type of add-in video cards you can opt for. Both physically and electrically.
I decided to drop in an ATI X1300pro 256 Mb DDR3 (from Dell). I was pleasantly surprised at it's performance. With a 600Mhz GPU core and 400 Mhz Ram it is very respectable for a low profile graphics card. Does a good job in Xp or Vista.
In fact I looked at the 7600GS and it appears lack-lustre in comparison to the X1300 pro. My recommendation: Don't be afraid to try the Dell ATI X1300pro. You may pay a little more, but you know, it fits, it works, surprisingly well too.
Tx. this has been my opinion.