Computers are not future proof. I would not invest in SLI because you can always get a card with dual GPU processors. Also the problem with two video cards they take up either a PCI or PCIe slot. I suggest getting the ASUS A8N-E. Until companies start pushing PCIe cards like IDE/SATA controllers, you have a 4X PCIe slot that can take high throughput devices.
The memory that you pick is way to slow for AMD Athlon64 processors. I suggest DDR433 because it can easily handle 400 Mhz and it will last longer. For games, I suggest putting at least 1 GB of memory.
You can try to wait until AMD Athlon64 X2 processor comes out in July or August. An AMD Athlon64 X2 3800+ is a good deal on performance and cost. Games have not yet to utilize multiple processor systems, but Linux schedules services on multiple processor systems better than Windows XP. You can make a CD/DVD, encode video, encode audio, or anything while you are playing a game. However you need an SMP kernel.
BTW, Next year AMD will be introducing another socket that makes this processor obsolete. It is known as the M2 socket.