With no experience with this particular title, I can only advise you based on your newbiness.
Your graphics card can handle most any sensible resolution. This is still likely to vary a bit, but using native for the best speed should not really be necessary. I would guess 1024x1024 would never be a problem, but some games are very heavy on the GPU. So, I suggest you try native if all else fails, or if the GS% on the PCSX2 window's title bar is at or near 100% whilst the game is slow.
Your RAM will do fine. It's not that important for PCSX2, just as long as you have enough. 2 GB or more is plenty.
Now, the most demanding (hardware) aspect of PCSX2; it's CPU usage/requirements. Like I said, I don't know how hard fullspeed is to achieve with AC3, but I'm guessing it may be possible for a C2D @ 2.66Ghz to pull off some playable speeds (with Speedhacks). Assuming you're already using the latest official PCSX2 release & plugins pack, you should try all [Recommended] Speedhacks
. Also, the EE Cyclerate and VU Cycle Stealing can do wonders for FPS, but beware; there's always a chance that too much of one of them, the use of both or even their use at all can cause glitches, false FPS display or even worse performance. As with the two sliders, anything beyond that is a "trial and error" approach on your part. What works/helps will vary from game-to-game.
If all else fails, you could try some things know to work with other games and see how they affect AC3. I didn't see anything about AC3 (recently) with a quick search of the forums, so it may very well be up to you to find out!