Dell XPS 410 PSU

Not entirely sure if a standard ATX PSU will fit in your DELL, but if not this VGA PSU would power your 8800GtX

http://www.newegg.com/Product/Prod [...] 6817153069

Almost any standard ATX unit will work, you just have to be sure to not purchase one that is to deep for the box.
The Dell psu is 140 cm, you could go up to 160 cm without problems.
Also the Dell psu is taller than standard, so if an aftermarket psu is installed there will be about a 1.7 cm gap below it, easily covered by tape or left alone, as it create's no issue's.
Do yourself a favor and pick up a couple of 80 mm fans to place on the rear grill, they will help keep temps down.

Hard to make a reccomendation, not knowing your intention's.
Do you ever plan to build your own ? you'd want something more powerful than you need for your current configuration.
In any case you would want something with at least 2 pcie power connector's.
Normally I would suggest the Antec EA500 because of it's low cost and quality, but they have changed oem's and there are no new reviews as to it's stability.
This Corsair 550 would do
http://www.zipzoomfly.com/jsp/Prod [...] e=10006757

Or if you prefer modular the HX 520
http://www.zipzoomfly.com/jsp/Prod [...] ode=373100

Fans are easy, two 80 mm's will fit on the rear grill or a single 92 mm, make sure they have 4 pin molex connectors.
Here's a shot of my 410
http://contrvlr.no-ip.org:12300/us [...] pgrade.jpg

No experience at all with Coolmax, so I won't touch that part of your question.
Yes, the GTX require's 2 power connector's.