However, I wouldn't push that PSU too far. If 400W is the minimum required, I wouldn't want to run it on a 400W PSU for very long. You'd essentially be pushing your PSU at 100% of it's capability all the time while gaming. This will create a lot of heat, and stress for that PSU. That could lead to a fairly short lifespan.
It's best to upgrade to a better PSU so it's not running at 100% capability.
The other thing to consider, is whether that PSU is rated @ 400W max output, or is actually capable of 400W consistent load. Those two figures are not always the same.
BTW: If you're using a 23" monitor, I'm assuming it's capable of 1920x1080?
If that's the case, the 4650 or 4670 isn't going to be your best bet for playing MW2 or COD5 at higher resolution or graphic settings. If you can afford it, I'd upgrade the power supply and the video card. You can get into an ATI 4870 or ATI 5770 for pretty cheap these days and those will give you fairly good performance for those games.