I wouldnt get EZ drummer if you can afford SD2. You dont need to sit and construct everything from the ground up to get good sounds, great presets are already included.
I highly recommend you goto at least 8 gigs of ram, if not more.
I had 8 and found it not enough. I use X2 with Superior Drummer + Metal Foundry and both Roots Expansions and made several custom kits. When you start creating custom layouts with multiple room mics, you can eat up ram awfully quick. A few of my superior kits is well over 2gigs of ram just by itself to load up.
I tend to run gigantic mixes with Superior running with Kontakt and several soft synths and large sample libraries, but Superior even by itself can get big.
Its well worth it by the way. I cant express how much more efficient I am now creating music instead of relying upon real drummers :). Its the best tool in terms of streamlining the creative process since gigasampler hit the scene 15 years ago.