I've got a fairly simple question, i'm quite hopeless when it comes to computers.

Could anyone tell me which computer here would be better for playing games?

As far as I can tell the only big difference is the processor. Now the first is dual core and the second quad core, and naturally as a newb I assume the second would be better for gaming. However I've been told that the first would be better.

Also, would the motherboard make a difference?

The first:

AMD Athlon 64 XP 6400 Dual core
Asus SLi motherboard
Geforce 8600 GT
2GB DDR2 RAM
500 GB hard drive

The second:

AMD Phenom 9600 processor
Gigabyte motherboard
Geforce 8600 GT
2GB DDR2 RAM
500 GB hard drive