Looking to upgrade Video Card; Power and CPU/Mobo-bottleneck concerns.

I'm currently using an ATI Radeon HD 2600XT video card, and am looking to upgrade to something a bit better, without building a completely new system. My current problems are:

PSU: I'm using an Antec SP-400 power supply, and most of the "good" cards I'm looking at in the $100-130'ish range require at least 450w, and some over 500.

Mobo: My motherboard's PCI-E slot runs in x4 mode (http://www.asrock.com/mb/overview.asp?Model=775Dual-VSTA&s=775) which was perfectly fine when I got it, since I was still using an old AGP card and it left me some room to upgrade. Is this going to bottleneck newer cards to the point that it isn't worth upgrading my current one?

CPU: I'm running an Intel Core 2 Duo E6300, which may also end up choking some newer cards, I would think.

I was hoping to put off upgrading my Mobo and CPU until the Core i7 comes down in price (substantially ), but I'd like to get a new video card now, unless I'll be bottle necked too much.

So far I've been checking out things like the ATI HD 4850, and 4870; and the NVIDIA GTS 250, and 9800 GTX+, all of which would seem to require a new PSU (going by wattage requirements I've seen on Newegg and manufacturer websites).

Could my Antec SP-400 handle any of those cards? If not, are there any cards with similar performance/price that my PSU would run?

Also, it seems that Newegg is having a nice sale on some Corsair and OCZ PSUs right now, which makes it pretty tempting to go ahead and upgrade to a 600-700w unit, along with a new video card; but will my Mobo and CPU bottleneck me enough to not really make that worth it?

The primary goal for me is gaming. I don't necessarily need to run every single game at 100% max settings with huge resolutions, but I would like to at least be able to get above disabled AA/Anisotropic Filtering and low/medium settings in Arma2