NCIX is like Canada's NewEgg: lots of stuff and fairly cheap (and fast!) shipping.
C$190 Asus SLI Mobo: http://www.ncix.com/products/index.php?sku=15495&vpn=A8N-SLI%20PREMIUM&manufacture=ASUS&promoid=1051
A64 3700+ $C250 CPU:
For C$440, you get an SLI mobo with decent onboard audio and you should be able to easily overclock the 3700+ to at least 2.7Ghz on stock cooling, which will smoke anything from Intel. You then buy the two SLI video cards separately and see how it goes. It should be able to play just about any game with decent framerates. However, if it doesn't, you put the 3700+ on eBay and get one of the dual-core CPUs, although in all honesty until games come out that actually take advantage of dual cores, you won't see much improvement. And because of hard disc thrashing, you don't want to be running anything anyways else while gaming. Video encoding might benefit from X2 (dual core), especially if you want to use your computer for something else while its doing it.
Also, you should know that a new generation of CPUs/mobos is just about ready to ship (last week in May for AMD). The AMD AM2 CPUs will have an on-die memory controller that supports DDR2, and Intel's Conroe isn't that far off either, and it looks like it might just take back the performance crown from AMD. Early sneak previews show that AM2 won't improve on existing AMD performance all that much.
If you don't want to go SLI (that can easily run you C$1200 in video cards alone!), you might want to look at this mobo as it can overclock that 3700+ CPU all the way up to the chip's limits (which will be entirely dependent on the luck of the draw; some weeks AMD makes CPU batches that can OC insanely, other batches barely above the stock speed):
DFI C$140 mobo: http://www.ncix.com/products/index.php?sku=13825&vpn=LP-UT-NF4-ULTRA-D&manufacture=DFI&promoid=1051
NCIX pricematches; if you find it cheaper elsewhere, they'll match it.
Don't forget you'll also need decent dual-channel RAM and a pretty hefty ATX 12V v2.0 power supply. Getting a pre-matched pair of 1GB sticks (ie. from OCZ) saves you trouble down the line. These two items alone will easily add C$300 to the price, and you'll still need a video card(s).