For monitoring your CPU temps try Core Temp, it works with Intel CPU's and should work with your AMD too. AFAIK it does not require your motherboard to support temp monitoring since it gets the value directly from your CPU.
http://www.alcpu.com/CoreTemp/
For your RAM you are correct, you must have sticks in pairs to run dual-channel. Dual channel can be picky tho, preferably you should get all matching RAM (usually sold as dual-channel kits). You can get 2x1GB kits for 2GB, or 2x2GB kits for 4GB. If your RAM is not matched, then you can run into serious stability problems and can adversely affect your performance with respect to timing mismatches. Also you will find generally 2x 1GB RAM to be faster than 2x 2GB RAM because of the higher latencies of the higher density chips in the 4GB kits. This is most noticeable if you are overclocking. If you arent then its not a huge difference. Usually you can run 2x 1GB at Command 1T, but 2x 2GB at Command 2T, and this has a performance impact on your performance, most noticeable with AMD CPU's.
I cannot directly answer whether your motherboard will accept a 6000+, in theory it should be no problem since there is no difference in bus speed that I'm aware of, just uses a higher multiplier. Perhaps there is a BIOS flash that will upgrade your board to accept this CPU? Try googling your motherboard model?
As long as your sound card is not right nect to your video card (at least one slot between) there should be no problem with heat with an 8800GTS. Of course.. more space is always better
