The first thing to note is that both games won't be supporting Windows XP, which has just passed its tenth anniversary. You'll need to be rocking Windows Vista, Windows 7 or Windows 8 in order to install and run either game.
In terms of hardware, you'll require a 2.66 GHz Intel Core2 Duo E6700 or 3.00 GHz AMD Athlon 64 X2 6000+ processor or better. Far Cry 3 demands a minimum of 4Gb of RAM, with 6Gb the recommended amount, while Assassin's Creed III will apparently run with 2Gb of RAM, but 4Gb is recommended for that game.
As for video cards, both support anything from the ATI Radeon HD and Nvidia GeForce GTX ranges, so long as they are 512 MB DirectX 9.0c-compliant with Shader Model 4.0 or higher.
Naturally, the higher above those specs your gaming rig goes, the better the games will look and play. If you really want to show off, the high performance specs for Far Cry 3 suggest Intel Core i7-2600K or AMD FX-4150 processors, 8Gb RAM and a 1024 MB DirectX 11-compliant video card with Shader Model 4.0.
Assassin's Creed III arrives on PC on November 23rd while Far Cry 3 follows on November 30th. If you're really impatient, Assassin's Creed III hits Xbox 360 and PlayStation 3 three weeks earlier on October 31st.