Some of the advice and warnings i've seen on here scares me alot.

Forget about the 8800GTX/GTS640/GTS320, they're outdated and obsolete.

You will not be satisfied with the 8600GT/GTS, it does not perform good enough to make a notable difference from your old one.

The advice offered about the PSU is the most worrying here. The 8800GT or Radeobn 4850 are both actually fully capable of being run on a 400W PSU. And no, it will not explode in your face if it's not up to the task unless DELL has seriously messed up (which they can't afford to do in their XPS line of machines. Last time I checked, DELL use quite good PSU's, and as long as it has the PCI-E 6 pin power cable that most modern graphics cards require, it's going to be alright. The worst thing that can happen (counting out the rediculous "blow up" and "catch on fire" scenarios) is that some parts won't power on, such as harddrives or CD-Drives (they are usually the ones that give up first, since they don't rev up until quite late in the boot-up sequence, and they draw alot of the power when they do, pushing the PSU off the edge).

My advice to you is this. Find out more about your current configuratiom. post more detailed info so we can help you easier. What CPU have you got? motherboard chipset? current video card? What kind of budget do you have? And most importantly, is there a 6/8 pin PCI-E cable loose from your PSU? Looks like this: http://www.techaddicts.net/reviews/da750/da750.15.jpg with either 5 or eight pins. Without it you probably won't be able to use a modern video card without an extra adapter, that requires two Molex connectors (the ones that are used for optical and harddrives) to convert into the 6pin connector.

Phew, lengthy post. Please ask if anything is unclear to you, and I will get back to you with appropriate links to reviews and/or relevant Wikipedia articles.

!!!!!!UPDATE!!!!!!
We opened up my computer (and put it back together properly too, I'm posting from it) and found a number of things:
1) My PSU puts out 375 w DC - so it needs to be replaced
2) My computer appears to have one PCI-E x1, one PCI-E x16, and one PCI-E x8 - so I cannot SLI
3) My computer has a lot of room, The video card could be up to: 12+ inches long, multiple inches high, and 4.5 inches wide - so space is not an issue.

Those prices seem a little low. Ordering the parts new from Dell when I bought my new XPS, two 8800M GTX 512 MB cards and a PhysX processor set me back $1000 OVER the price of a 256MB NVIDIA card of a type I don't remember.
Obviously, desktop cards are cheaper, but I highly doubt you could get a new 8600 series card for $50 bucks, and DEFINITELY not an 8800 series for $150, my 8800s were $500 each...

Hey, I have the same exact Dell XPS 410. I wish to upgrade my computer's Video card also. I noticed rafi 117 upgraded his video card to the latest ATI 4850. I PMed him asking how the installation of the 4850 and the new PSU went since I'm planning on doing the same. No reply from him for days. Does anyone know to how get in contact with him?