With the sale on the HP2-6400 2x2GB at ncix, those are going to be a great choice for 2x2GB.
I would grab those personally, they are no slow ram, they are tested well beyond rated and warrantied upto 2.1v
This allows for normal overclocks of 1000-1066Mhz 2.0-2.1v 5-5-5-(12-15), worst I've seen was 950Mhz, but that was a p965 chipset and that was expect, when not using auto.
Your board should have no issues running them 1000Mhz+
The HP2-6400 2x2GB will most often run 800Mhz 4-4-4-12 also.
Mushkin HP PC2-6400 4GB 2X2GB DDR2-800 CL5-4-4-12
For a nice heavier heatsink, you might want to look into our ascent line, even with 2.25v (2.35v actual on my board), I couldn't make them warmer then air temps, with nothing but normal case cooling. Same for 2.35(2.45 actual on my board) using a 120MM fan over them.
Mushkin Ascent PC2-6400 4GB 2X2GB DDR2-800 CL4-4-4-12
No problem about wanting to try another ram, I went though many different ram's before sticking with mushkin for my last ** builds. My first kit was the xp-4000 2-2-2-5 2x512MB 3.5-3.7v from way back, was soo fast.