You have two main things to worry about: 1) RAM. This will determine how many mods, of what size, you can append to the game. Skyrim, after the 1.3.10 patch, supports 4GB of RAM. You'll need an extra 2 GB on top of that to keep the rest of your PC running. :) So, 6GB RAM in total. 2) Your GPU/graphics card. The more that happens on the screen, the more load this puts on your card. The more detailed/profuse/OTT your mods, the more you're going to stress your card.