Exactly sirphoenix, there are a few things in a PSU you should watch, and that is the 12V-rails (amperage), if you're going for a high-end card, as some low-end PSUs don't have enough 'oomph' in those. (Get a Corsair 850W, it has a single, massive 70A 12V rail - that's A LOT ^^ watch the heat buildup though, good ventilation is always important.)
And just as mentioned, high efficiency is damn much more important than the numbers the manufacturer put there. (*cough* Q-Tec *cough* - just an example, some of you know what I mean ^^)
Look for the "80plus" rating, then you know you've got a decent one.
Also, use a power meter (plugged in between your computers power cable and the wall outlet) to see how much your PC really is consuming. My rig;
OS: Windows 7 Ultimate 64bit build 7000
PSU: Corsair TX750W
CPU: E8600 Intel Core 2 Duo 3.3ghz
GPU: Gainward GeForce GTX 280
RAM: Corsair XMS2 Dominator PC8500 4GB (2x2GB)
MOBO: Gigabyte GA-EP45-DS5 (Intel P45)
HD: 1x Samsung F1 500gb, 2x Samsung F1 1tb
Consumes just around 100-150W idle, (depends a lot on power savings settings and active devices) and up to 350-400W peak, thats far below the 750W (even with the .80 rating = 600W) the PSU can handle.
And yes, PSUs can degrade a little (up to 30%, though far from all do this) over the years, but as seen you don't need as much as you might think.
Also, a PSU is quite a lot more effective if you live in a country with 230VAC, compared to 110-115VAC (see http://www.corsair.com/_images/charts/tx750w_efficiency.jpg), so... yeah, move to one
There is one thing I don't agree with sirphoenix on though; "the higher the clocks the better you're sins experience will be." - The way modern processors work this is not the case, overclocking may increase the performance of a CPU, but you will never reach the theoretical increase in power you would expect. (i.e. an E8400 @ 3ghz clocked to 4ghz will NOT perform 33% faster, due to how a CPU and data transfer between it and the Northbridge is handled - the effect depends on how you clock aswell, using multiplier, bus or both.)
I'd say, go for a fast dual-core instead for a quad-core at the same price, as nearly all applications today are optimized for one, in some cases two, cores. Yes there are exceptions, but if you do not know whether you need a quad core CPU, you probably don't. Just get a quad core if you can add 50% of the money you'd have spent on a dual core.
Oh, and to the OP:
I'd say your system is definately well as it is, and I would advise not to change anything. (Remember that overclocking may damage your components, and you'll never in hell see your warranty if your components have been clocked - other than from factory. Same thing goes if you change your graphics card cooler (better cooling for clocking) for example, warranty void in nearly all cases.)
Just a random tip as well, make sure your memory modules get the voltage they should have (around 1.9-2.0V on the XMS2, check your memory specs.), instead of the DDR2 default at 1.8V, to ensure maximum system stability and performance.