Im Upgrading my pc with a new Gpu and i was wondering how noticeable would be
the bootleneck, taking in count that im Planing to buy a gtx 570 or even a 670 if i get the money, ill be upgrading the cpu too but ill have to wait 6 - 7 months
Overall you should be fine. A bit of a slow down for any game that needs a lot of CPU power. I wouldn't be to worried however. As mentioned if you can OC it then great, but a quad at 2.66GHz should be fine for most/nearly all games.
Indeed, a q9400 will be adequate for a gtx670 plus, plus mentioned I would recommend against a cpu upgrade for a while instead invest in a 212 (mere 20 bucks) and oc that boy, but most games will be not botlenecked by that cpu, and if so the overall fps would be above 60 for all games on medium or high and thus adequate.
I'm running a GTX570 with my Q6700 oc'd to 3.2GHZ. I've estimated my bottleneck to be between 5% and 10% depending on the game. I must have run a dozen benchmarks. The bottleneck isn't really even noticeable, save for a few hundred futuremark points.
Your Q9400 should meet or exceed this overclock, depending on your cpu's vid and your cooling budget. That being said, an I5/I7 has architecture and efficency improvements that speed up most computer tasks. My 3dmark scores may be in the same ballpark as a similar spec'd machine with an I5, but my Linpack results are not in the same universe.