I think you need to check your second RAM there cause no OS other than a 64bit OS can use more than 4GB of total system memory. Unless that is supposed to be your HD space which plays very little into performance.
Your best upgrade is to build a new system as the components in your current XPS 400 are limiting and very out of date already. Your PD is already 4 generations of processors old and you can get alot of performance from upgrading that. Your also going to be limited on the RAM as your going to need to find older ram which isnt as fast as newer ram.
You need to figure out your budget first, what you want is pointless unless you have the money to spend on it. Your 8800 GT will cost around 200 and some additional ram will cost you anywhere from 50-150 depending on what you have currently in your system and if you need to replace any of it.
(If you have two slots with 512 each and you want to get to two gigs then your going to end up buying two 1 gig sticks. If you have four slots with two 512 sticks and you want two gigs you can buy either an additional 1 gig stick or two 512s.)
There are plenty of threads in the general section with builds and suggestions for builds with parts listed from newegg. I would start going through the back posts and figure out which path is better for you:
Either spend money on an old and outdated machine
or
Spend money on a new freshly built machine new parts and some salvaged from your old system. (New mobo, processor, case, PSU, RAM, & vid card)