so what im asking is should i get a 4gb kit or 2 2gb kits? is there really a difference between 4 gigs and 6? and last will i see good performance vs buck if i crossfire a 4850. i cant really find any benchmarks right now for it.
If you are using a 32-bit (x86) Operating System, you do not need any more RAM. After you add another GPU, you will have at least 3+GB total memory. The 32-bit OSes cannot see and use more than ~3.25-3.75 GB of RAM. If you are using a 64-bit (x64 bit OS), adding 2 GB RAM may help you performance, get the 4GB (2x2) Patriot RAM.
The GPU: If you can find a 4870, get that. If you cannot find one, the 4850 would the next best.
I must add that between 1x4GB and 2x2GB, i would go with 1x4GB because it requires one slot,so its easier to upgrade later.And the difference between 4GB and 6GB isn't much,because most games/apps will work fine with 4GB RAM.
As for VGA,well CrossFiring a HD 4870 with a HD 4850 will give you the performance of 2 4850's,which is good, but i recommend to get another 4870 because its better to use 2 same cards in CrossFire mode.