I belive the Dell XPS 600 is an SLi system. What CPU do you have? A 7900GS isn't too bad but it won't cut it if you want to see high, or even medium in some cases, settings on todays games. What resolution is your monitor? Since you are likely to be bottle necked by your CPU you may not see the benefit of the 9800GTX+ over an 8800GT if you're gaming below, and perhaps even at, 1600 x 1200. Since it's an SLi system you should think about picking up another 7900GS from eBay.
If you just want to get one new card now then you should think about when you want to upgrade your system. You can put in a 9800GTX+ now and use that card in your next computer if you want to upgrade it in a year or so. If you'd rather just build a whole new computer or want to keep this system for a while, then just get the 8800GT and be happy . Since your system should be using DDR2 memory I would get the 9800GTX+ now, upgrade the RAM, Heatsink, and Motherboard in a few months to overclock the hell out of that Pentium, and then Drop in a new CPU a few months after that. Since it should be an LGA 775 CPU you shouldn't have a problem putting it in a new board. Of course, always check that boards supported processor list to be sure it will accept what you have ^_^.
And its a crappy Pentium D. An E2180 would eat that cpu for breakfast and is cheap but I dont think that mobo supports it. If he replaces the mobo then he has to deal with the operating system installation issue.
This is why I hate prebuilt systems. If you buy them to use as they are then you will be fine, usually. You run into so many limitation issues when you try to upgrade and its sickening.
Unless you get a super deal on the 9800GTX+ and plan to build a new system soon I would stick to buying the 9600GT or 8800GT, they both will give you a decent performance boost for the money and play most of todays games pretty well. If you could go to a core 2 duo cpu you could really unleash the video cards capabilities but you might as well build a new system if you go that route.
Going SLI with a 7900GS wont give you as much performance boost as 1 9600GT or 8800GT. You are better off upgrading the video card IMO.
No prob and enjoy. My friend bought one and runs a 32" on his desktop and another 42" on his bedroom wall to watch movies and tv. We played F.E.A.R. and it was very smooth gameplay. You wont be disappointed.