Lani, I skipped CS4 completely. Some of the other software in the Master Suite caused me so many dramas I stuck with CS3. I've now got CS5, and although there is nothing earth shatteringly new in there, it has enough that it is worth the upgrade. However, if you've not long bought CS4 I would stick with it and get your monies worth.
CS5 runs like a dream on my 3 year old
Dell 1420 laptop with 64-bit Win7 Ultimate. Basic specs are: Core2Duo 1.66GHz, 2.0GB RAM, Intel on-board video.
It runs equally well on my work machine; Core2Quad 2.40GHz, 4GB DDR2, Leadtek Quadro video, WinXP Pro 32-bit.
I would highly recommend buying the biggest SSD you can find. PS uses the page file heavily and a quality SSD makes more difference to the speed of this program than RAM or CPU.