I took a look at the motherboard specs you have. That 8400 is a PCI-E card, and the other card you want to put in is also a PCI-E card(or it could be an AGP model). Your motherboard only has 1 PCI-E slot according to this
, which is already taken up by the 8400. It also doesn't have an AGP slot either incase that x800 was an AGP version.
So you won't be running the both of them together. Really though you can't run an ATI card and Nvidia card together, unless; you use the Nvidia card for rendering Phsyx(all8xxx series and above NV cards can do this, and only some games utilize this) and the ATI card doing everything else.
So no, you can't. Another problem too would be the power supply you have, pre-built computers tend to have a sub standard power supply in them.
Here's two links you should check out though for your general knowledge. SLI Using 2 nvidia cards together, you can also get Tri-sli with 3 cards running together ATI Crossfire, their version of 'SLI'
Edit: Also, those 2 card themselves aren't that great so the performance increase wouldn't be that big.
Edited by IrishGrimReaper, 02 November 2009 - 12:54 PM.