GTX 460 768mb should be all the power you need for that resolution. Great card, great price and a 500W PSU will easily handle it (though I cannot comment on the actual quality of the PSU you have as I am not familiar with it)
Can u guys tell me if i can run the GTX 560 Ti on this PSU and if not how about the GTX 550 Ti ???
Yes, either one will run on a 500w PSU - 500w is the recommended minimum so more would be better, especially since that is a low quality PSU. I could not find a review for that exact model - NRP PC503 - but here is one for its brother - NRP PC502:
What is the difference between an old graphics card and a new graphics card of the same series, say GTX 560 Ti and GTX 460 ???
Generally speaking, newer cards have later technology and features - such as DX11, are built with smaller circuitry that makes them more efficient and run cooler, and, compared to the specific card they are targeted to replace, cost a little more and often, but not always, run a little slower. For the newer technology you pay a premium for performance relative to the older cards.
Regarding the GTX 460 cards, the 1GB version is marginally faster than the 768MB version, but not enough to make up for what has been a substantial difference in price with some steep discounting on the the 768MB version.
The GTX 400 series and the GTX 500 series are exactly the same, except for enhanced performance and efficiency. Their graphics rendering abilities are exactly the same, i.e. both are DirectX 11, PhysX, CUDA, etc.
I'm guessing the NRP-VC503 is slightly improved over the NRP-VC502 so it may be a decent PSU. Looks like the older model was a little short of that. Anyway, with that PSU I think you would be fine with a GTS 450, 550 Ti, Radeon 5770, or 6850 which is what I would recommend for your resolution. A 460 or 560 would draw more power and while that setup should be o.k even under load (my OC CPU with a single 5870 drew less than 450 from the wall with Prime 95 & furmark loaded) I think it would be pushing it a bit with that PSU.
I say just get a Radeon 6850, or 550Ti if you must use nvidia, and be happy. That should be more than enough for 1680x1050 for some time, though admittedly you may have to drop to med-high as the norm in a year or two. If you want the 560, 460 or better/higher power sucking GPU then I would certainly upgrade that PSU.
A reputable review shows it using about 57w or almost 20% MORE than the reference design, which is not surprising since it is overclocked. This review shows power consumption from many different brands and models of GTX 460 and the one you selected is near the top of the list in power consumption.
Modular PSU's have removable cables so you only install the ones you need. Non-modular are less expensive, but the unused cables are permanently attached to the PSU and need to be tucked out of way. It's a matter of cable organization and airflow that is easier to do with modular cables.