I've got two machines - excluding the video card, which I know about - which would be immediately better for gaming -
System 1
AMD Athlon 64 X2 Dual Core @ 2.5ghz
3GB DDR2 SD ram
Windows Vista Home Premium 32bit SP1
or System 2
Intel Pentium D Processor 805 Dual Core @ 2.66ghz
2GB DDR400 PC3200 ram
Windows XP Professional SP3
System 1
AMD Athlon 64 X2 Dual Core @ 2.5ghz
3GB DDR2 SD ram
Windows Vista Home Premium 32bit SP1
or System 2
Intel Pentium D Processor 805 Dual Core @ 2.66ghz
2GB DDR400 PC3200 ram
Windows XP Professional SP3