Which RAM should I get?

^^ Crucial Ballistix Sport - good choice. They make some with some pretty high speeds and pretty low CAS timings.

What you want generally is a memory set which has as high a speed, with as low a CAS timing as you can afford. There is a declining advantage with faster speeds vs price and the sweet spot right now between price and performance is in the 1600-1866mhz speed range and CAS timings around 9.

Why CAS is important. This is how many clock cycles between data fetches/writes etc and when the memory is ready for the next command. If you look at it, this is the difference between a CAS 7 and a CAS 11. The CAS 7 is 4 clock cycles faster than the CAS 11. BUT! Remember, that means that CAS 7 can perform 3 operations in 21 cycles, while the CAS 11 only performs 2 operations in 22 cycles. That puts the CAS 7 at 30-40% faster. For CAS 9 vs CAS 11, the difference isn't quite as much, but it still does things faster. CAS9 = 10 operations in 90 ticks, CAS11 = 8 operations in 88 ticks = CAS9 is about 20% faster.

Why MHZ is important - it's the speed at which the memory operates. Obviously, faster memory is better than slower memory.

BUT! You have to compare both MHZ -and- CAS timings. If you have 1600mhz @ CAS7 vs 1866mhz at CAS10, which is faster? General rule of thumb? Divide the MHZ by the CAS. This gives you a rough operations per second and in our example above, the 1600mhz CAS 7 comes out to a rough score of 228, the 1866mhz CAS 10 comes out to a rough score of (duh!) 186. So - the 1600 CAS 7 is technically a bit -faster- than the other memory in spite of having a slightly lower clock speed.

Right now, Mushkin Redline Enhanced is a 1600mhz CAS 7 8gb kit which should be downright quick for about 95$ USD through Newegg. There are also some 1600 CAS 8 and 9 sets which are more affordable.

So - general rules of thumb when figuring out what you want to get:

Anything faster than 1866 is not a good price/performance ratio at the moment.
Stay away from anything with a CAS higher than 9.
Divide the MHZ by the CAS to get a rough comparison between memory modules to determine which is faster.