Another vote for Crucial.com here.

BTW...it's my understanding that DDR2 RAM is not backwards compatible to DDR systems. In other words...if your system uses DDR RAM and that's all it's spec'd for, it can't use DDR2. "Mismatched brands" (i.e. Crucial running with Kingston, for example) is one thing...mismatches types is another entirely (i.e. DDR in a system with DDR2 and DDR+ECC). Mismatched types is a no-no.

As far as cost, etc. I always look at it this way: is it worth saving the few extra bucks if your system starts overheating, possibly leading to data loss? Invest a few minutes shopping the 'Net (Crucial.com, NewEgg, etc.) and find the decent memory at the right price.