vista! The requirements

processors:
intel pentium 4 2.4 ghz+, intel celeron d 2.6 ghz or better(I am not really sure about celeron)intel pentium m 1.6 ghz or better, amd athlon 2400+ or higher, amd sempron 2600+ or higher, amd turion ml-28 or higher.
memory and hard drive:
512 ddr 333 mhz or better, a 5400 rpm hard drive with 3-5 gig of free space
GPU's(probably the most questioned):
good news! Most intel GPU's with a 915 chipest of better can use windows vista!
ATI:
Mobility radeon x300+
Mobility radeon 9500+
Express 200m
reasonable fire gl graphics card
radeon 9500, x300+
all in one wonder x600, 9600+
radeon express 200 and 200 crossfire
Nvidia:
notebook,
GeForce Go 7800 GPUs
GeForce Go 7300 GPUs
GeForce Go 6800 GPUs
GeForce Go 6600 GPUs
GeForce Go 6400 GPUs
GeForce Go 6200 GPUs
GeForce FX Go5700 GPUs
GeForce FX Go5650 GPUs
GeForce FX Go5600 GPUs
GeForce FX Go5200 GPUs
GeForce FX Go5100 GPUs
desktop,
GeForce 7800 GTX GPUs
GeForce 7800 GT GPUs
GeForce 6800 GPUs
GeForce 6600 GPUs
GeForce 6500 GPUs
GeForce 6200 GPUs
GeForce 6100/6150 GPUs
GeForce FX 5900 GPUs
GeForce FX 5700 GPUs
GeForce FX 5600 GPUs
GeForce FX 5500 GPUs
GeForce FX 5200 GPUs
GeForce PCX GPUs
workstation,
NVIDIA Quadro FX 4500
NVIDIA Quadro FX 4400
NVIDIA Quadro FX 4000 SDI
NVIDIA Quadro FX 4000
NVIDIA Quadro FX 3450
NVIDIA Quadro FX 3400
NVIDIA Quadro FX 3000G
NVIDIA Quadro FX 3000
NVIDIA Quadro FX 1400
NVIDIA Quadro FX 1300
NVIDIA Quadro FX 1100
NVIDIA Quadro FX 1000
NVIDIA Quadro FX 540
NVIDIA Quadro NVS 285
NVIDIA Quadro NVS 280