Time to upgrade?

I currently have a HP Media Center PC (m1195c) with a XP Media Center Edition 3.2 Ghz HT Processor/2 GB RAM (I had upgraded)/250GB internal HD/400GB external HD/and a X1600 with 256 RAM that I had upgraded and a Dell 24 inch 1920x1200 monitor.

I am considering building myself a new system with a
Q6600 Quad Processor
2 GB Ram
Not sure on a motherboard
8800GTS 320 Ram
2 Dual TV tuner cards
Raid with 5 500GB drives in Raid 5.

I spend many many hours a day on the computer but spend most of it just surfing the web and IMing or watching video on the computer(want lots of space for more video). I do play some games though like C&C 3 and Oblivion but dont spend nearly the time with them as I do doing other things on the computer.

I have the money right now to upgrade, but do you think its worth me upgrading right now considering what I have and what Im doing with it or just holding off and saving for something better down the line? Also, Im not sure weather to put vista or XP on the computer if/when I build it. I tried vista business on my current HP but it ran WAY to slow and was not usable. Id hate to put all this money into a new computer with vista and only have it rn vista as well as my current computer runs xp, but at the same time it seems crazy to me to build a new computer and then put an old OS on it. What do you think?

Thanks a lot in advance

okay, thanks. For the amount of gaming I do its probably not worth the cost of a GTX though. Thanks for the suggestion.