Microstuttering can be serious, but it is very variable dependent. With low RAM and a slower CPU the chances of microstuttering can be greatly increased. It also depends on your motherboard, if it does not transfer the data from the PCIe slot quick enough it will also increase the chances for microstuttering. The monitor you use can also greatly effect the outcome as well. With high response times you can get ghosting and video lag (>8ms generally), and with very low response time microstuttering can become very apparent (2ms). My personal opinion is to go with the 4870, the 9800 GX2 will vary in performance a lot more than the 4870 would. The 4870 is STILL working on BETA-like drivers, and at 1680x1050 I would say with in a month or 2 the performance difference will be very low, and possibly on the side of the 4870. Also the 4870 requires less power and runs extremely cool with a simple fan mod. Not to mention your CPU will bottleneck the 9800 GX2 more than the 4870.
I would have to say either is a good deal, but the uncertainties about the 9800 GX2 make me side with the 4870. It runs cooler, requires less power, gets more consistent framerates, will be easier to upgrade later with another one, can only get better, and the difference in performance is really laughable in real life. The 4870 is more than enough power for all games, even Crysis which is a terrible game that you will not find yourself playing much at all, trust me. It is all up to you, but that is the decision I made myself.
Concrum, I am very against causing conflicts with other posters, but your just so pathetic, I simply can't resist. I want you to read every thing you have written in this thread and read your other anti-4870 X2 posts. Notice how microstuttering is only a concern when its an ATI dual GPU, not nVidia. I also want you to read how you keep bringing up the inconsistent performance of the 4870 X2, because it is a dual GPU, and I want you to see how you have dismissed it when its the 9800 GX2 in question.
OP, Concrum is NOT to be listened to, period.
Edit: My final verdict, go with the cheapest one.