Memory to go with 1gb 667Mhz

[masten]

Hi

Ive got 1GB of 667 Mhz GEIL memory (2 x 512mb) in an evga nforce motherboard which has 4 memory slots I think. Its also CAS 4 whatever that means

I want to put 2GB more in so whats the best option? Get 2 paired sticks of 1GB GEIL? or is it OK to mix manufacturers and if so whats good and good value, and does the speed and CAS have to be the same???

Reliability much more important than performance

What about these:

http://www.overclockers.co.uk/showproduct.php?prodid=MY-097-CS&groupid=701&catid=8&subcat=144

http://www.overclockers.co.uk/showproduct.php?prodid=MY-227-CS&groupid=701&catid=8&subcat=144

and whats the difference?

Thanks a lot