XPS 630 - Will I be happy

Sounds good, plenty of grunt for modern games. 4GB DDR2 800 is good there is faster (1066) but that really comes into play when overclocking. The 9800GT is pretty good but I am in love with the ati 4870. (Not sure about SLI settings) Raid isnt really necessary as the performance gain is hardly noticeable, unless you want redundancy (raid1?).
Check the PSU is a good un you want grunt, clean power delivery and preferably nice and quiet, same goes for your cpu cooler.
I would also get a better mouse, something like a razor.
Dell are pretty good and at least if it breaks you can send it back.
I built my own for £1200, It is very quiet and nice and grunty too.
I went for an E8400 (OC'd to 3.6), a Noctua NU-U12P CPU coooler, an ATI 4870, A 1TB HDD for data and a 500GB for OS etc, Asus P5E X38 mobo, A Coolermaster Silent Pro 700w psu, 2x2gb Crucial 8500 Ballistix and a Razor Krait (although I find it a bit small for my hand). Plus case etc and all from ebuyer who I find very good.
Whatever system you get im sure you will find it a massive improvement and will have no issues with current games, Enjoy And check out Left 4 Dead, im counting the days.......