Upgrade Dimension 9100

Hi, this is my first post.

I have a dimension 9100 that is getting old:  Pentium 4 (3.2GHz), 1GB dual channel DDR2 400 MHz (2x512), 2xATI Radeon X600 SE (256MB) and 320GB serial ATA RAID 0 Stripe HDD (2x160GB).

Also still running Windows XP.

I would like to upgrade RAM and video card especially to allow playing of more up-to-date games and am considering upgrade to Windows 7. Would anyone be able to advise me on best (and compatible) RAM, graphics card and whether or not Windows 7 is a good idea.

Many thanks for any advice given