I currently have 2 gigs on a 32bit OS system. I'm thinking of going to a 64bit and upping my memory to 4 gigs. I currently have some medium grade 1200 memory (1x2 running at 1066). Question is, can I just buy another 1x2 set of memory of any brand and stick it in the other dual channel. I understand I'll have to run timings and clocks of the slower memory. I'm more concern about the dual channel performance. I know it's best to have matching sticks in dual channel to get the most out of it, but does that apply to matching sticks across all the slots, or just dual? Should I just get brand new 2x2 memory?

More DIMMs puts more strain on your memory controller which can reduce the maximum clockspeed at which your memory and your FSB can run. The best solution is to buy 2x2GB. Especially given the incredibly low cost of DDR2 now.

Yea that makes sense. Any suggestions on brand any type? I'm looking for something on the cheaper side.

G.Skill is usually the brand of choice for "cheap but good". OCZ and Corsair are good too, but a bit more expensive. Prices have unfortunately increased pretty dramatically over the last couple months since the DRAM market is rebounding, but you can still get a good 4GB kit of 1066 DDR2 with decent timings for about $80 on Newegg. I'd recommend this low-voltage kit from G.Skill but note the large heatspreaders may interfere with CPU heatsink fitment, depending on distance between CPU and DRAM sockets and the size and vertical clearance of your CPU HSF. If you're worried about fitment, go with this kit or this kit instead.