Wow, thanks everyone!
I knew you can swap power supplies in a home brew but was afraid to mess with the Dell, it might be designed to need that supply. Glad to hear I can upgrade it without hurting anything! I have put 8 gigs in the supposedly 4 gig max machine and it does pretty well.
As to not getting the 8400 GS, I know it's limited, but anything better wants even more power, more than my current PS has to offer. So I didn't think I could go there. I need a solution tomorrow, are you guys sure it's OK to go with larger PS? If so I'll go down to the local shop and get both PS and graphics card.
Do I really need more than the 8400 GS? I don't game, or do anything demanding. I just run my 26 inch Samsung Syncmaster, 1920x1200, for office stuff, internet, programming.
Thanks again, I appreciate your help.