i'd like to speak my 2 cents (which will end up being more like 200 in length)
the Athlon64 3200+ is not a dead equivalent to a Pentium 4 3.2GHZ
in gaming it will step ahead and compete with the 3.4 or the 3.6
and in video endcoding it will fall behind and compete with the 2.8 or the 3.0
It's a great processor, considering it competes with the Pentium 4 range with great ability
as for graphics interfaces
AGP is inferior to PCIe, and the 6600GT for PCIe is a wise choice, a little overkill for basic gaming, still a wise choice
AGP is under half the bandwidth of PCIe
and is half-duplex (this is what makes PCIe so great)
half-duplex means it can send and recieve, but it can only do one at a time
so it would be like this:
Send - Recieve - Send - Recieve
and so on, it's not bad given that graphics cards don't have much data to send back to the computer
here is how PCIe would look
send and recieve - send and recieve - send and recieve - send and recive
and it has it's full bandwidth each way
meaning if it were going 2GB/s, it would have 2GB/s to send, and simultanious 2GB's recieve
while AGP @ 2GB/s is very simmilar, it cannot do both at once
AGP also supplies less voltage (which was fine for/is fine for cards that use AGP, but newer cards are needing more and more power)
PCIE also is more expandable (current machines only use 16 lanes, or 2 8 lane distrobutions for CrossFire or SLi, while PCIe can support up to 32 lanes, and 2 16 lane configurations (or 4 8 lane, or any other possibility)
PCIe also provides more bandwidth to the graphics card (interface bandwidth) which means demanding games like Doom 3 (which utilizes full AGP 8x (only game to ever do such)) get a few % more performance on PCIe over AGP, as their not hitting the ceiling of interface bandwidth
PCIe is just "bigger, better, faster and stronger" so to speak
I hope you enjoy your new machine, and your new 6600GT
if you'd like to see how it stacks up to modern graphics cards, and see how it stacks up in modern games
check this out
http://graphics.tomshardware.com/graphic/20041222/index.html
and the later test (which I personally do not like using, as some of their game choices make you ask what they are thinking)
http://graphics.tomshardware.com/graphic/20050524/index.html