Recently I installed AC Brotherhood. It is completely unplayable even at 1024x768 res with everything on the lowest or turned off. i have played AC2, AC:R and even AC3 before on the same PC. 3 and revelations ran at about 40-30 fps while still looking nice(mid settings). I maxed out 2 while still running at 50ish fps. I am using the integrated Intel HD 2500 with Core i5 3470 and 8gigs of ram. I tested both in Windows 7 and Windows 8.1( which I installed today) And both have the same results. I also played all the above- mentioned games at native 1440x900 (excluding Brotherhood, which I played at 1024x768)
You're going to need to get a graphics card, the Intel HD 2500 is not suitable for playing modern games, it's designed to display your desktop and provide video playback. Intel's HD 4000 IGP is much better, though still likely wouldn't perform well with Assassin's Creed, however, the HD 4000 is only available on certain desktop CPUs, and the i5 3470 is not one of them.
If you want recommendations on which graphics card to get, please post how much money you are willing to spend. Details about your power supply will also be helpful, particularly the amps on the +12V rail.