The MHz rating is the same, and they should both be compatible in the same PC.
I'd look at the manufacturer site and see which one has better timings
(e.g. 5-5-5-15 vs 5-5-5-18 or 4-4-4-12 or whatever.. lower numbers = better).
On the other hand you can get a 2GB stick of faster memory (DDR2-800)
for like $53 US if your motherboard can take that capacity DIMM. For expanding
memory two of those for $100ish would beat two or four of the 1GB dimms
in speed and overall benefit.
http://www.ewiz.com/detail.php?name=T8UB2GC5&show=p