I think Vista is better for Direct X 10, I read something about it but I am not an expert, if you are looking to play new games from what I read Vista is the way to go.
Go to Dell, HP or Gateway or where ever and get something similar to this
http://configure.us.dell.com/dellsto...WDC3&s=dhs
PROCESSOR Intel® Core™2 Duo Processor E6550 (4MB L2 Cache,2.33GHz,1333 FSB) edit
OPERATING SYSTEM Genuine Windows Vista® Home Premium edit
MONITOR No Monitor edit
MEMORY 2GB Dual Channel DDR2 SDRAM at 667MHz- 2DIMMs edit
HARD DRIVE 320GB Serial ATA Hard Drive (7200RPM) w/DataBurst Cache™ edit
OPTICAL DRIVE Dual Drives: 48X CDRW/DVD COMBO Drive + 16X DVD+/-RW Drive edit
VIDEO CARD 256MB NVIDIA GeForce 8600GT-DDR3 edit
SOUND Integrated 7.1 Channel Audio edit
I am not recommending this particular system but this is kind of a baseline.
Some sites are really overcharging for RAM upgrades, so watch out. $100 for an extra GB is about double too much.