Long answer: You can't "mod" a mobo to be SLI. It either IS SLI or it's not. For SLI you need 2 PCI-E X 16 slots (although they both become X 8 when you use the SLI feature if I am correct) and then you need an ethernet bridge to connect the tops of the 2 cards.
Short answer: If your mobo didn't come with the bridge packaged with it (it is like a 2 inch by 1 inch piece of mobo material with a connector on each end) then your board cannot support SLI.
Aspire X-Dreamer case
Thermaltake Silent Pure-power 480W
P4 3.2Ghz (OC to 3.7Ghz)
1GB Crucial PC3200 DDR SDRAM
XFX 6800GT AGP (OC to 390Gpu / 1100Memory)
160GB Seagate 7200Rpm HD
Audigy 2 5.1 Sound Card
Plextor DVD Burner
56X CD/DVD-ROM drive