I currently have 4GB (2x2GB) of DDR2. I was planning on building a whole new system (the old one is going to be a hand me down) and just going with the DDR3 rather than buying new DDR2 since eventually I'll need to buy it anyway.
As for overclocking, I'll probably try and max out the cpu on stock cooling and then scale back a little for everyday use. I'm currently running an E6600 2.4Ghz at 3.4Ghz.
I'm going to be running a Radeon HD4870 as well, no crossfire or SLI. Honestly, I've always been more of an AMD fan, but I didn't see any AMD boards with DDR3 yet so I was going to go with an Intel Quad Core. Even with Intel processors I've never been a fan of Intel chipsets in general, but I haven't used one in many years.
For my new setup I've been looking at the following:
Quad Core Processor
Motherboard with DDR3
4GB DDR3
HD4870
WD 300GB Raptor (maybe 2 in RAID 0, current system has 4 Raptors in raid, 2x36GB and 2x72GB)
This is all just after messing around on newegg for a little while, I haven't been keeping up with all the new stuff for the last couple years so I don't know what's good and what's not right now.