SR2038X RAM Upgrade: PC2-5300 or 6400, Compatibility?

Question

A quick question, probably dumb but i'm skeptical as to if i can get some PC2-6400 RAM for my computer. I've got the stock, that came with my Compaq SR2038X(2 x 512MB). I want to upgrade to two sticks of 1GB for 2 gigs. Crucial says 6400 will be compatible, but the Compaq page says 4200(currently what i have) and 5300. Is there any reason to suspect the 6400 to not work w/ my mobo? Also, i think i could use the RAM i currently have, and that could up it to an overall ~3GB of RAM, although then all of the sticks will still run at the original 4200 speed. Is that better or worse than simply just using the new 5300/6400 ram for 2GB?

Crucial says: http://www.crucial.com/store/listparts.aspx?model=Presario%20Media%20Center%20SR2038X

HP/Compaq says: http://h10025.www1.hp.com/ewfrf/wc/document?docname=c00757531&lc=en&cc=us&dlc=&product=3262013

And can anyone recommend good RAM? I was looking at this:
http://www.newegg.com/Product/Product.aspx?Item=N82E16820146526

Answer 1

2x512Mb is fine for XP and 2x1Gb is fine for Vista. PC2-6400 will work. However if your motherboard only supports up to PC2 5300, BIOS will just downclock that PC2-6400. Mixing different types of ram might disable dual channel memory mode. Some applications requiring wide memory bandwidth might actually run slower on single channel mode even if you have 3Gb total RAM.

Answer 2

You are correct that all of the ram will run at the lowest speed installed, so if you are going to keep the 4200, you might as well buy 4200. It is sure to be cheaper, no use throwing money away.
The larger amount of ram should more than make up for the difference in speed.

Answer 3

Basically, any PC2 should work on motherboards with PC2 standards, the numbers point out the peak bandwidth of the memory in MB/s. So the lower the number, the smaller the bandwidth. It will work slower, but it is compatible.