I would go with an i7 processor (maybe the 930) as you get hyperthreading.
On HT enabled programs, you can extra virtual cores or threads. As in the quad-thread just turned into an 8-thread. Threads are kinda like cores. They allow multiple tasks. A single core can perform one task only, but if it has 2 threads it can do up to 2 tasks.
AMD does not have hyperthreading, as Intel has patented it. So whatever the core count on an AMD is…that is the thread count. The core i7 series by intel is amazing, as it boosts by having a possible 8 threads (depending on model)!
So, I suggest a Core i7 930, 6-8 gigs of ram (In previous version of premiere I have gone over 4 gigs easy…but I have not had the issue with CS5 yet..so this is up to you). I suggest the ram be DDR3 133mhz or 1600mhz, which means you need a DDR3 board.
https://www.newegg.com/Product/ProductList.aspx?Submit=ENE&N=100007627+600007912+600036384&QksAutoSuggestion=&ShowDeactivatedMark=False&Configurator=&IsNodeId=1&Subcategory=280&description=&Ntk=&CFG=&SpeTabStoreType=&srchInDesc=
Those are boards that support DDR3 (1333mhz and 1600mhz) as well as in i7 (that takes the 1366 socket).
As far as video card…stay with nvidia. I would recommend something a little more powerful though, depending on the scale of your video you are working with.