How many Watts PSU should I buy?

500-550watts

Either one of those PSUs would be sufficient for your system and a mild OC. Do you already have the GTX 460? If not, then why not go with a better card? If this will be primarily a gaming rig and money is an issue, then I'd recommend scaling back on the CPU and upgrading the GPU. Going with an i3 2120 won't cost you a performance hit because your current build is GPU limited. That will save you an extra $70....the extra $70 will put an HD6870 or HD6950 in your budget and that is a nice upgrade.

I would get at least a 650 watt PSU. having plenty of headroom is always a +1 your system will thank you for.

A GTX 460 requires a 450W PSU with 26A on the 12V rail and (2) 6-pin PCIe power connectors. Amps on the 12V rail is more important than the amount of watts. Actual power consumption of a GTX 460 system at full load will be around 300W using a power-hungry i7 965 with a serious OC: http://www.guru3d.com/article/geforce-gtx-460-review/13

The same system with the HD6950 I recommended will draw about 361W at full load: http://www.guru3d.com/article/geforce-gtx-460-review/13

I personally use an HD 6950 and an i3 2120 in my rig with a Seasonic M12II-520W and it is rock solid at full load. Do you need more power than the OCZ ModXstream 500W or 600W can provide? No and unnecessarily going overboard on the PSU costs money that could be used on other components. Do I recommend higher quality PSUs? Yes. Seasonic PSUs are on sale right now as are XFX PSUs (made my Seasonic). I'd recommend stetching the budget for a better quality PSU, but you're looking in the right rating range.

I already have the gtx 460. Because it won't be primarily "just for gaming", I'll keep the 460 for a while then sell it and buy another one later. Need a bit more money first though  

Completely understand upgrading like that. I'd still recommend getting a better quality PSU. The Seasonic/XFX PSUs on sale are better PSUs, cost nearly the same as the OCZ PSUs...you just don't get the rebate.

Which OCZ 500w ?
Not that it really matters as long as it has dual 6 pin connectors
Your system, with it's low power cpu, will draw a great deal less power than the OC'd I7-965X system in the review rugger provided

System power difference of I5-2400 v Stock I7-950 , based on full cpu load

from ;
http://www.xbitlabs.com/articles/cpu/display/core-i5-25...

Over 45w difference at idle

42w idle difference here ;
http://www.bit-tech.net/hardware/cpus/2011/01/03/intel-...
with a 6970

Both of the above reviews use less efficient power supplies than guru3d, meaning the same systems power consumption would be lower with the guru3d psu

You have nothing to worry about, other than idle efficiency, with any decent 500 watt psu

The only OCZ PSUs on discount are the ModXstreams...decent PSUs.