man, ram is so cheap, just get that OCZ 4gb kit either new or from the other dude, its worth it. I have that exact kit, I am more than happy with it. It overclocks to DDR1000 very easily, I don't know why you would not be interested in overclocking when its as simple as dialing up extra performance.
If you are going through the trouble of dual-booting XP and Vista for the very minor performance difference it offers, changing a single value in the bios with with a clockable kit will get you way more performance advantage than the difference in the OS's.
Get the 4gb OCZ Platinum, install it, clock it to at least 900, install a 64 bit OS, and you will see a REAL advantage. Take for example the recently released cinematics mod for Half-Life, it will only run on 64 bit operating systems because of RAM adressing issues with the 32 bit OS's, simply because it uses more than 2.8 gigs of ram. You might not care about that particular game but it sets an example of the direction things need to go in order for games to look better.
I think you are making very small upgrades (2gb single chanel to 2gb dual channel ; 8600 to 8800), and you are looking at products that are already on their way out, you would be much better off upgrading more aggressively. I only say this because RAM and video cards are both pretty cheap right now, it's a great time to be buying DDR2 and midrange video. Vista LOVES ram. The more ram you have, the better Vista's prefetch can work, which means more of your programs will start up instant-like.
6 months ago I bought 2gb of OCZ DDR-400, it was $150 or something, mail in rebate
1 month ago I bought 4gb of OCZ DDR2-800, was only $89, no mir, @ ncix
last week I received the $20 rebate cheque from the old ddr-400 ram, already used and sold ;)