Essentially the only thing you need to worry about when buying new RAM is what speed (i.e. the Front Side Bus e.g. 400 MHz, 533 Mhz etc) your motherboard supports. Some points to consider:
- If your RAM does not match the speed of your motherboard, it will run at the slower of the two speeds, or might not possibly work at all.
- If you mix a faster speed RAM, with a slower one, both will run at the slower speed of the two RAM sticks.
- Its preferable (but not required) that you use two of the same size RAM modules. This can result in higher efficiency and performance.
- To be on the safe side, its best to not mix different brands of RAM as it may possibly result in incompatibilities. However, more often than not, it'll run just fine.
The short answer to your question is, yes you can use Kingston 800Mhz and Corsair 800 Mhz RAM together, and it should work perfectly.