3GB is plenty and running at 333mhz will give you a bit faster performance most of the time.
I have like a 4 year old computer, it originally came with two sticks of ddr2 ram each 256 mbytes and each 266mhz. i have bought new ram over the years and now i still have those two plugged in along with a stick with 1024 mbytes at 333mhz, and another stick of 2048 mbytes at 400mhz. I was wondering if it would be faster to take out the two original sticks, i would lose 500 mbytes of ram, but it would only go up to the 333mhz, Is it worth it? will it be faster to leave them in or take them out? (I have the dell dimension e510)