Dells are pretty good psu wise we get a lot of posts like this.
The cards you are talking about draw all their power from the motherboard via the pcie slot, the amount off power supplied is standard so any card like the ones you quoted that do not require an extra power supply ie molex or six pin pcie power cable will run in the slot.
The main thing is what do you want to do with the system if its gaming then you need something different as the card you mention is more suited to a HTPC and i would recomend a 2600xt for that use anyway.
Let us know what you want to use the pc for and we will be able to stear you right
Are you running Vista or XP by the way?
I did a fair bit of research on the 2600xt and the 8600gt and it looks like with the new drivers (and cards) these two boards are about equal in all things. Thus, if you prefer Nvidia or ATI you really can't go wrong. Otherwise, consider the bundle that you can get with the card and go with the best value (or whichever will offer your favorite game if you can find it).
I'm getting this version, seems pretty good for it's price.
Calibre 8600GT, PCI-E, DDR3, 512MB,Overclocked Version, w/HDTV, Dual DVI the review looked pretty good. http://www.bjorn3d.com/read.php?cID=1132
If you can marty, I would consider saving yourself some money and getting one with 256mb of ram. I have often heard that less powerful cards (ones with a lower bitrate like the 128 the 8600 uses) do not take advantage of the extra memory. Even in some tests an 8800gts 320 performs the same as the 640 version due to the fact that in a very few cases the extra memory is not needed. The 8800's are very powerful and have a 320-bit memory interface. That's nearly 3x the bitrate. I am not saying that card is bad, just a 256 will perform in all reality just as well as a 512.
I,ve been reading the reviews of the 8600GT cards for a while now, and the review I've read on this card is pretty good. Temps are ok and performance for money (in Australia) is great. out performance the Leadtek OC card and the 2600XT in most charts and in some test gets close to the 8800gts. It's also almost the same price and in some case's cheaper the the Stock versions of the 8600gt. (At least in Aussie).
For my needs it hits the sweet spot between performance that I need and price.