um brand name is pretty important as far just as the quality of the ram goes. If i've never heard of the company thats selling the ram i wouldnt buy from them.
DDR is double data rate of its core clock.
For example, the first DDR RAM to come out was DDR SD RAM, which was just SD RAM (133mhz) double pumped (x2) so that made is 266mhz.
its a good idea to match up the core clock of your ram (DDR speed / 2) with the FSB of your processor, so they both run at the same core clockrate.
CAS Latency has to do with the time it takes the ram to fetch information, so the lower the better, heatspreaders are pretty pointless unless your going to be
overclocking like crazy. even then, in my opinion, they are just a geeky fetish.
a good ram choice would be a brand name, eg. kingston, corsair, micron, infineon, with a core clock that matches your FSB and a CAS 2.5 for value and 2.0 for performance ram.