You won't find a Athlon 64 + Good Motherboard for under $200. It'll be like $230 without shipping.
HOWEVER, the Sempron 64 processors are the little cousins of the Athlon 64s. They are more powerful than the Athlon XPs and not that expensive.
As far as the motherboard goes, the DFI Lanparty UT nF3 250 Socket 754 Motherboard
is the best motherboard for Socket 754. It also has a AGP slot for your 6600GT. Its $99.
The AMD Sempron 64 3000+
is the best processor you can get without going over your budget limit. That processor is 1.8GHz and the only thing that seperates it from the Athlon 64 3000+ is that it has 128KB L2 cache. The Athlon 64 3000+ has 512KB L2 cache. This processor is still great and its an AWESOME overclocker. I've seen people take this processor to 2.7GHz on air with stock cooling. The processor is $89.
That comes out to $188 without shipping. Shouldn't go over $220 with shipping.
The Sempron 64 is a 64-bit processor (hence the "64" in its name) so it will run any 32-bit operating system like Windows XP pro, and it will also be able to run Windows XP pro 64-bit edition, and Windows Vista when it comes out in late 2006.