Bottom mounted PSU?

Yes, it will work fine, at most I'd pick up one of these just in case the power won't reach.

http://www.newegg.com/Product/Product.aspx?Item=N82E168...
$5 Athena Power 8" Extension for the ATX 12v

The ATX12V cable on the GS600 is 60cm. The NZXT Guardian 921 is 45.9cm tall, so there will be no problem with that connector reaching where it needs to go.

This is the better buy as it is more efficient and is cheaper after rebate. It also has four PCIe connectors, so is more suitable for a multi-GPU build.
XFX Pro 650W Core Edition 80Plus Bronze $90 ($30 rebate, $6 shipping)
http://www.newegg.com/Product/Product.aspx?Item=N82E168...

Yes it might be no problem at all, but check the steps in the following article
http://www.tomshardware.co.uk/cooling-airflow-heatsink,...

That's the one I've been reading - great article.

But the PSU I linked has a top mounted fan intake, and the case is bottom mounted.
Theres a picture of how not to mount the PSU, upside down.

But I've read in aother forum that it dosent matter which way is up. So you can flip the PSU so the air intake faces downward.

But they also said that it dosent really matter at all which way the air intake faces, because the case temp will never get high enough to damage the PSU in anyway.

Just make sure that the case has holes for ventilation in the bottom, so if u face the fan downward you get no problem.

The PSU fan is important, it takes the fresh air inside the case, cools the capacitors and coils, exhaust it out of the case.