I highly suggest one of the DFI SLI motherboards. They can be had for $170-200 at Newegg. I will elaborate on the differences between the three versions if you're interested but you really should research them yourself (this is always preferrable, since you really ought to be a fan of the brand/product that you are buying). In short, they are great for OC'ing and gaming performance. I suggest -- who else -- Anandtech for furthur details. Note that with this kind of motherboard, it's a real waste not to get high-quality RAM. I'd suggest (512MB x 2) OCZ Platinum Revision 2 or a dual-channel kit of DDR500 from the same company or a like brand (see the Anandtech article on unofficial support for DDR500 for Revision E AMD64 CPUs for more details).
For the CPU, you could wait for the $350 AMD64 x2 if you feel that you need that extra CPU performance for multitasking, encoding, or rendering. Otherwise, you could probably go for the 3000+ or the 3200+ and buy a custom heatsink from Zalman, Thermaltake, Swiftech, or another like brand to OC your CPU to higher performance.
EDIT: The 3700+ is a good choice if you want that blazing fast speed without OC'ing, but I really think buying a higher clock speed for AMD64 CPUs is silly if you get the DFI since it and the CPU overclocks so easily. That's just my opinion, however, and it's slightly slanted because I'm willing to OC. Not everyone is, and building a computer is highly personalized.
Remember, a gaming computer should be built around the GPU, so make sure you're getting a GeForce 6800 GT or higher (or the ATI equivalent) with that kind of processor and motherboard.