The speed of the RAM depends on the "MHz" reading after some numbers. The bigger that number the better. You should always install RAM in matching pairs for best results. Please use this site: http://crucial.com/systemscanner/loader.aspx?Agree=Yes
Crucial will tell you what type of RAM your system currently uses, and the clock speed, "xxxMHz" (xxx being any set of numbers). The bigger the number the faster the RAM will be at processing data. The size (512Mb, 1024Mb/1Gb) is just the capacity of the RAM stick.
i think 2*512mb would be best if u have dual channel ram. even if u don't, your mobo would "see" more ram (2 is bigger than one), and therefore slightly improve performance. having two ram chips will also improve failure levels (if one stick fails, u still have 512mb left!!)
as Groox said, consider adding 2*1024 or even 2*2048 (if options exist to do so for your configuration). Ram isn't that expensive, but is a major speed criteria. If you want to run vista, it will eat lots of ram, and the more u add, the better ur vista situation. if sticking with xp or below, 1gb should be more than enough.
In single channel mode, one larger stick will provide slightly better performance than two smaller sticks because the memory controller only has to address one slot. The difference is extremely small, but real.
I'd suggest someone study up on memory controllers.
It is a Compaq Persario using DDR (double data rate).
Not using Dual Channel, RAMBUS or other 'exotic' flavors. and still will only achieve 2700mbps. There are no synchronus/asynchronus modes for that model, if there were, the sockets would have come filled, and would be listed as Channel A/B rather than DIMM1/2.
Even the higher end r3000 & r4000 didn't use Dual Channel.
Been studying, and have studied over 20 years as well as designing and moding