This is a very difficult question to answer at this point, and you are absolutely in the right timing to ask it (confused yet?...hehe).
After returning my 6th in-store i5 replacement, I decided to sit this out, but I am unsure for how long. Like man others are saying here, the updated iMacs should be ariving by May or June. That's a mere 3-4 months away. Although the update will be minor (loosely based on Apple's track record on iMacs). We should expect a little bit of the hardware update, but the exterior should stay the same. There's also a talk of a touch screen iMac but this just boils down to user preference. I like touch screens, but they get tiring.
Couple this with the issues these iMacs are having, and I'm not only talking about yellow screens, dead pixels on the iSight camera and buzzing sound, I warn you of another issue that is not wide spread (or should I say not immediately noticed by people): faint small greyish speck inside the LCD (not in between the glass and LCD but embedded on the LCD itself). My 6th replacement was a week 5 i5, and it had this defect, as well as the 3 other replacements before it.
We still do not know if the week 6 iMacs are all defect free at this point. I suggest skimming through these threads for updates.
My answer to you: Wait another month or so, if you really need to have the iMac now. You won't really regret it when updates arrive since they will be minor. Also, do monitor these threads for reviews of the current stock of iMacs (week 6 onwards) before pulling the trigger.
This is where I am right now.
I hope this helps.
