Your
Biostar TA690G supports up to 8Gb of DDR2 memory (max of 2Gb/stick) running up to 800Mhz.
Look into a JEDC standard DDR2 800 set with CL5 timings running at 1.8v for best compatibility.
What are the CPU and GPU you are looking to upgrade from?
What is your budget and intended usage?
You should have no issues running any current GPU on your motheboard but you will be very limited on your choice of CPUs.
Your motheboard only
Supports Older Athlon 64 x2 CPUs up to a maximum of the 5600+.
Unless you are still running an older single core CPU, you would be much better off upgrading the motherboard and CPU together.