What are you trying to do? Upgrade to 2GB or 4GB?
As already suggested, it best to keep the sticks matching.
I would also agree with Hopalong.
If upgrading, do away with the old RAM, and replace with matching sticks in both
channels so you can run in Dual Channel mode.
With only 1 stick it will be in Single channel.
DDR2 800 will be perfectly fine. (DDR2 is the type of memory it is ... And the 800Mhz is how fast the memory operates. You board supports up to 4GB running at 800Mhz)
The only reason to get memory faster than that is if you are overclocking for the extra headroom. But you also need a motherbaord that can support the faster frequency.
OCing on the Motherboard you have will not be a good idea, so the 800mhz RAM is more than enough.
I would purchase a matched kit thats either 2X1GB (2GB total) or 2X2GB (4GB total)
Heres a 4GB Kit for $70: http://www.newegg.com/Product/Produc...82E16820231122
And a 2GB Kit for $40: http://www.newegg.com/Product/Produc...82E16820231098
Granted many different other brands and price ranges depending on the budget, but an idea at what your looking at.
My advice would be to would go with the 4GB matched kit.