You'll be happy with both options. I personally own a phenom x4 at 3.5ghz and am really happy with it, including with titles like starcraft II which are cpu-heavy and only use 2 cores. I also game at 1920x1080.
Personally if I had to choose right now I would go for the i3, though. You'll be satisfied with it. As for multitasking, the cpu supports hyperthreading (each core has 2 threads) which may not come close to 4 actual cores but each sandy bridge core is stronger than phenom so it's still alright. You also have a stronger upgrade path as you can get a very strong 4 core sandy bridge and it's almost certain that ivy bridge will incorporate socket 1155 so you'll be able to upgrade to that - At 1024x760, though, I really doubt it if you'll need to.
But if you can afford to wait a couple of monthes tops, I really think you should. Bulldozer is right around the corner and even if it'll disappoint it's still going to be somewhat competitive and drive prices down.
Personally if I had to choose right now I would go for the i3, though. You'll be satisfied with it. As for multitasking, the cpu supports hyperthreading (each core has 2 threads) which may not come close to 4 actual cores but each sandy bridge core is stronger than phenom so it's still alright. You also have a stronger upgrade path as you can get a very strong 4 core sandy bridge and it's almost certain that ivy bridge will incorporate socket 1155 so you'll be able to upgrade to that - At 1024x760, though, I really doubt it if you'll need to.
But if you can afford to wait a couple of monthes tops, I really think you should. Bulldozer is right around the corner and even if it'll disappoint it's still going to be somewhat competitive and drive prices down.