Originally Posted by nleksan
I think that, with space taken into account as well as your use of the APD2 and intent to cool a 580 in the loop, there are a couple really great choices that will give you truly excellent cooling both now and when the GPU is added, and won't overwhelm your pump (yes, the Apogee Drive II is the MCP35X pump but it does suffer a bit of flow due to the fact that it is fitted to the CPU block, and thus some concessions must be made). The Phobya 1080 is beyond overkill for what you are looking for performance-wise, and I don't think that you would see a bit of difference with such low heat loads. It's a great radiator, but I would question its benefit in a system like yours.
I would highly recommend getting at least a 360 (3x120) or 420 (3x140) radiator, as it will be enough to allow for decent headroom for overclocking (for a CPU+GPU loop), although I think that a 420 or 480 would be perfect, considering the significant difference between the 5xx and 6xx cards' heat output.
Here's what I would be looking at:
1) Alphacool NeXXos UT60 420/480 (or 360 if you really don't want anything larger) - the absolute best radiator platform, bar none, over the greatest range of fan speeds; all copper construction makes a significant amount of difference in performance, and while it's 11fpi allows it to perform above almost every other radiator at fan speeds from 600-1200rpm, it is far from being a "low speed rad" only as above 1200rpm, performance scales at a nearly perfectly linear rate, outperforming the competition (some rads are fairly close, most aren't even near). Truly the absolute best radiator when it comes to versatile performance without compromise. The 420 would be my pick, but otherwise the 480/360 if you really want to see its potential with super fast fans.
2) XSPC EX420/480/360 - incredibly impressive radiator, and the most versatile slim rad, so much so that it is almost indistinguishable from the rads twice as thick except for one way, and that is the fact that it can usually outperform all but the very best of them. The 19 split fins per inch essentially double the surface area, while keeping the restriction level as low as possible. The result is a radiator that does quite well at slow speeds, but pulls ahead of the pack above 1400rpm or so, and stays there. At very high speeds, it can even pull out in front of the UT60 (above 3krpm), but the NeXXos still has lower flow restriction, perhaps the only weak point of the EX series.
3) HWLabs Black Ice SR1 420 - the ultimate low-speed radiator that has a reputation for keeping pace with a number of "high speed tuned" rads. For a quiet yet extremely capable system (as in, you will never go above 1krpm), this is an excellent, albeit expensive, option. If you are going to be spending more time with your fans at a bit higher speed, the UT60 420 is a far better performance:dollar choice.
The RX360, Phobya G-Changer 420, and the like are also good rads, but I feel that the 3 above are the absolute best all-round performers. The UT60 excels at any speed, while the EX prefers to have 1200rpm+ fans and the SR1 is the best for sub-1000rpm use. I recommend 140mm based radiators as the Bgears Blasters 140mm 1800rpm fans are very good, actually they are excellent, and they undervolt extraordinarily well while still having more static pressure than any other 140mm fan, and for $9/ea. Considering the 420 rad has MORE surface area than a 480 rad, and has only 3 fan hub "dead spots" instead of 4,they are the superior option IMO.
I would go push pull regardless, as it will be of some benefit (exactly how much, I can't say for certain). You may also consider using shrouds for at least the push fans, both for the possible performance boost and the noise reduction, but that's up to you. Personally, I would use something like a 3x140 7-12mm thick rubber gasket/shroud/decoupler on the push side, and on the pull side either no shroud but use some weatherstripping or double-sided tape to create a seal, or at the most useful some Phobya 7mm thick shrouds/decouplers.