Some things you also should think about:

1: When you say upgrading do you mean that you will fill your old system with some new stuff (GPU CPU, Mainboard, RAM) or will you buy a complete new PC? In the first case, be careful, the system has to fit in your old PC (that may sound odd, but ir isn't trust me!) and your power system has to be strong enough to take it.
2: Even if you buy a new system you should care about whether your system components fit together or not and whether you can place them together or not.
3: When you buy a new mainboard, choose it very VERY carefully!!! In my experience the mainboard is the most important component in a PC. Check the hardware forums out there for reviews and experiences made by others and also check whether the mainboard had problems with certain components (eg. GPUs so far)
4: Think about temperature. I myself have to downclock my GPU as it otherwise would be to hot for my system. Choose your GPU having this in mind and inform yourself about the coolingsystem of the GPU you want and also watch the textreviews, that show temperature behaviour of the GPUs. If you want to use an old system and want to upgrade it with new components think about buying some extra cool down systems or "system blowers".