I'd personally recommend a Phenom II x4 (I have a 965 myself and it runs like a tank) on an AM3+ platform. You can upgrade with no problem once Bulldozer comes. The thing about Ivy Bridge is that it's going to be expensive. Really expensive - as are all of Intel's premium platforms. AMD's total platform cost almost always seems to be lower on average.
Right now, my Phenom II x4 965 can handle anything I throw at it. I game at 1080p with a Radeon HD 6870, and it's easily more than enough power for me. No, I can't max out the anti-aliasing on Crysis at the highest graphics settings, but I don't really care. The difference isn't even noticeable, and it's just not worth another $1000 to me.
The Core i3 can be overclocked the old traditional way by increasing the frequency, but by no more than 10% - 15%. I believe the USB ports are tied to the frequency so if you increase the frequency too much that would stop the USB ports from working properly or possibly damage them.
It's difficult to say how bulldozer will perform, but the general consensus is it will probably not be faster than a current Sandy Bridge CPU. We have to wait for actual benchmarks.
A stock Core i3 is probably as fast in games as a Phenom II X4 OC'ed to 4.0GHz - 4.2GHz. Therefore, if you are gonna buy the Phenom II X4 955, then you would need to overclock it by 800MHz to 1.0GHz to equal a stock Core i3 in games. It would have to overclocked even more if you OC the Core i3 to 3.3GHz or 3.4GHz.
My advice is to buy the Core i3-2100 and overclock it by 200MHz or 300MHz and upgrade to Ivy Bridge when it comes out and when you have enough money for it.
I have to say go for the AMD set up and have fun OCing it to 4GHZ, it will easily keep up with any single card GPU you care to pair with it, put it all on an AM3+ Mobo so your future proof and ready for Bulldozer. Like someguynamedmatt said, intel is just not worth the extra money unless you need to power some super high end dual gpu set up to play at insanely high resolutions.
Why do I get the feeling that Intel are butchering AMD at every corner. If a dual core by trade can smoke a quad and even a hex core, that to me suggest that AMD need to start focusing a little more on processing power over the litle gimmick called 3D now. Seriously who runs a 2500k with integrated graphics anyway?
you don't even need to wait till IVY if you go the i3 route: if you want to upgrade to stomping multithreaded performance, you can upgrade to the 2500K or 2600K ANYTIME you want to, as they are available now and will still beat bulldozer.....
and of course you can then still go with IVY when it comes out, if you really want to have a screaming system!
just pointing out that there is already a massive upgrade path for that i3 you are considering.
upgrading aside, i went i3 over the X4 mainly due to the heat issues. i live in the middle of Hell, texas, and it's just too freaking hot here to have a space heater sitting next to me all day.... i also like knowing i'm saving a few bucks on my electric bill too.
I am one open to change, but really the question is simple, Have AMD got any answer to the Intel high end processors (even mid level)?. If a company brings out a processor aptly called a bulldozer, and it literally gets bulldozed by a sandybridge low-end CPU, the future looks very bleak.
I would hate to de facto resort to intel just because.
It just seem like AMD are playing their hand early with as early set release date, Intel will by and large match with the revised Sandy Bridge, Ivy Bridge will hardly raise eyebrows, but going on the status quo, it hardly needs to be good to maintain intel market dominance.
It is sad though, personally I find AMD very intriguing, but am unwilling to sacrifice pure processing potential for so-so graphics based CPU enhancements. If AMD were to legitimately rival Intel at high end they would be very attractive. The llano for example is very fascinating but lacks the bang necessary to really move one from Intel to AMD.