As pappy mentioned, some of the pre-built Dell systems (and other brands too) come with the video "integrated" into the mother board, and no AGP or PCI-e slot available to upgrade. Your potential for upgrade will depend totally on whether your motherboard has an available slot for a graphics card. Even if your MoBo has a PCI slot available, (you can find many cheap PCI video cards now), you're looking for a PCI-e or AGP slot, not plain PCI.

Might be worth a call to Dell support... they seem to know what they're talking about when you give them your service tag number, etc... they might have some suggestions, OR they might just try to sell you a new computer!

EDIT: I've just looked up the Dimension E510 on Dell's site... I assume you've not done much (anything) custom to the machine? The specs actually look fairly decent, except for the video card part... "Hypermemory" is ATI's code word for shared vRAM and system RAM... BAD NEWS for sims. Hypermemory will slow down your graphics every time. I would give Dell a call, give them your service tag so they'll know exactly which machine you have, and tell them you want to upgrade the RAM and video capability of your machine. The worst they can do is not help...

-Case