If it's a Dell Dimension E510 and not modified in any way, then you are currently using integrated graphics
(open it up and check, just to be sure)

blurb here: http://reviews.cnet.com/desktops/del...-31555136.html

Specs here: http://reviews.cnet.com/desktops/del...html?tag=specs

you have one PCI-E x16 slot, so you can upgrade to pretty much any card out there

you have a uni-core processor

so if you're planning to upgrade to vista, you may be disappointed by the performance

the choice of card is up to you, but I think your processor is going to end up being the bottleneck

You currently have 512mb ram
you might also want to think about bunging in a lot more

some cards:

High end
http://www.dabs.com/ProductView.aspx?Quicklinx=4BG0

Mid range
http://www.dabs.com/ProductView.aspx?Quicklinx=4R4D

Budget
http://www.dabs.com/ProductView.aspx?Quicklinx=4KPG