when shopping for RAM look at the timings provided on the specs page for the ram. To get great benchs you are going to want the lowest latency possible. The all time best RAM ever, no longer available though, were brands like the Mushkin BH5 that ran at 2-2-2. Those have long since sold out.
Since you are going to be running DDR2, you have a much higher limit to the speeds you can get. Look for timings around 4-4-4-8. The value RAM will be closer to 4-7-7-12 or something. (just examples) See the difference?
Look at RAM such as the mushkin High Performance 2GB DDR2 533 with timings that are 3-2-2-8. Really good stuff, a tad pricey at 325 for the Dual Channel but you will definitly love the headroom you got. This will set your system free and get rid of any limits due to your RAM.
There is better stuff out there but this is the best in teh practical range. You could drop 800 on some ridiculously insane RAM at 1000 mhz but thats overkill
Also check this out...probably even better: http://www.newegg.com/Product/Produc...82E16820145564