Upgrade or wait (does involve OCing)?

I'll probably end up make this longer than it needs to be but here's my dilemma.

Some friends and I were talking about building new systems and I decide I didn't really need to build a new one anytime soon but I did decide to upgrade mine a bit.

So here's what I've been running for about 2yrs now:

CPU: AMD Opteron 165 @ 2.5Ghz (stock 1.8Ghz) (runs 40C idle and less than 50C stressed, all air cooled)
RAM: 2 x 1GB OCZ PC3200 Ram w/ 2.5-3-2-5 1T
Mobo: Asus A8N32-SLI Deluxe
V-Card: 2 x nVidia 7800GT's in SLi (stock speeds)
S-Card: Creative X-Fi Fatal1ty Gamer
HD: Western Digital 74GB Raptor 10,000rpm AND Maxtor 250GB (both SATA)
Rom: Plextor 16x DL DVD-Burner (also SATA)
PSU: just replace Hyper 580w (blew out) with Zalman 750w
OS: XP Professional 32bit



And here's what I just bought for it

RAM: 2 x 1GB OCZ PC3200 Ram (same exact model as what I had before to bring the total to 4GB system mem)
V-Card: nVidia 9800GTX+ (only 1)
OS: Vista Home Premium 64bit

And here's what the problem is, I guess I didn't do enough research before buying this stuff cause I cannot get the 4GB of ram to work with my overclock spd. Actually I only wanted 4GB because I'd heard from people at work and online that if you want to run vista well you should probably have 4GB as it hogs a lot of the ram. I haven't tried the 4GB at lower CPU speeds yet, as I'd hate to be losing performance.

So I guess my question is what do you think I should do here? Run Vista with just 2GB? Try and run Vista with 4GB but at a lower CPU speed? Send it all back and just wait to build a new system (earliest would be mid summer 09)? or any other ideas?

On a side note the overclock I've had for the last 2yrs I haven't messed with the memory timings at all just cpu frequency. Also with replacing the PSU I recently tested (before I bought the more ram and stuff) that I can get it up to 2.8Ghz but it's not Prime95 stable but I haven't had anything crash on me at those speeds but I don't know enough to figure out how to get it Prime95 stable at those speeds so I lowered back to 2.5Ghz which is Prime95 stable for 24hrs+