Hmmm, I'm also stuck between the two. I run my desktop at 1680 x 1050, on a 22" monitor, so not sure whether the 4890 warrants the extra money. Definately needs a new graphics card, as most won't display at full screen at desktop res (black screen - I THINK it's because the current graphics card (HD 2600 XT 256mb) only outputs at 60hz). But 4890 has less power consumption than 4870. Dell Vostro 400 from last year has a 350W PSU I think, but little space in the case. Bah, when did PCs get complicated... I want my Amiga back.
By the way, if you struggle to work out if your PSU can run a graphics card (or any other upgrade) I use
eXtreme Power Supply Calculator as a handy guideline.