CHKN
and btw, if u gona buy a new psu, dont hesitate to buy a good brand, corsair, thermaltake, ... it may cost more, but a quality psu is the base of a good rig
What CHKN said. You can get cheaper ones, but you get what you pay for and a cheap one can blow your system. Get at least a 750 watt model or better. 64 bit OS will let you use all of your memory, but don't get this unless you plan on upgrading to 4 or more Gb. nVidia 460 is a good video card at a great price, as is the new 550. 560's jump up in price from the previous two, so if budget is a factor...
As Merlin said, your platform is still great for current games, so no rush for a new rig just yet. Adding a better video card will make a world of difference, as will more memory. If you haven't overclocked before and are interested, do a pile of research before you jump into it. www.overclockers.com has great guides in their forums, as does www.overclock.net. Overclocking is NOT something that you can jump in to and do quickly. Slow and easy wins the race there.