AM2 is also a dual channel DDR2 platinum, meaning it needs two DDR2 sticks to operate at max speed. You'll have to stick in two DDR2-800 sticks to acheive peak performance. DDR2-667 and 533 will work, but you will take a noticeable performance hit in a lot of apps.
But, if you're going to stay with the GF6600, you might as well save some more money and go with the 667 or 533 RAM. X1900XT 256Megs are around 250 USD. Nvidia flavors can be found for less, a 7600GS is around 90USD and a 7600GT is around 140 USD. Any of those cards will outperform the 6600, with the X1900XT obviously giving the greatest amount of headroom.
Just a heads up, I have the MSI K9N SLI Platinum board. MSI puts a tiny little heatsink on the northbridge, which gets very hot in a very short amount of time.
It hasn't effected my stability at all, but its something to be aware of.
It would help to know what you're going to use the system for.