Which PSU is best for my system?

If you think you might want to pick up ATI's R600 part then go with the Silverstone Olympia 650W as it has the correct 8 pin PCIe connector, if not then just pick whichever one you feel most comfortable with.

Ta. Will do.
I'm not planning on going R600 any time soon, but I think a couple of others have the 8 pin PCIe connector too. I'll look into warranties.

Do any there look like bad deals for their prices, relative to the others in my we list?

It's not just that it has an 8 pin PCIe connector but that it has the correct one. Several of the PSU's floating around that claim they have an 8 pin PCIe connector are in fact using an older version of it before the latest PCI-SIG standards were approved so do be careful of that.

US$ Prices using Newegg.com

Thermaltake Toughpower 750W - $189.99
Enermax Liberty 620W - $169.99
Enermax Liberty 500W - $114.99
Corsair CMPSU-620HX 620W - $169.99
Silverstone Olympia OP650 650W - $149.99
Silverstone SST-ST75ZF - $199.99
Silverstone SST-ST65ZF - $139.99
Silverstone SST-ST56ZF - $137.99
Antec NeoHE 550W - $109.99

The only 2 that Newegg doesn't carry are the Zippy model (most likely because Zippy is primarily a server PSU company) and the OCZ model. Prices look to be fairly similar in structure, besides the obvious increase due to exchange rates and such. Looking at the prices closely I'd say even though you don't plan to pick up an R600 based card the Silverstone OP650 is probably going to be the best choice, rather it's the one I'd choose if I were in your position. The only concern might be that it isn't modular so if you have a smaller case with barely adequate airflow it might not work out so well.

On a side note, I think it's pretty interesting that Newegg has only been carrying the Silverstone Olympia OP650 PSU for a little more than a week and it's already sold out.  

Any particular reason that I should get the Olympia not the Liberty 620? Or the Corsair 620 for that matter? The Zippy 600 and the Antec are a bit more expensive for what you get. I'll ignore the 500 models I think, along with the Toughpower and the enhance built Zeus(personal preferences there).

Zippy/Emacs GSM 600W - $356 -- $---.-- ---
Corsair CMPSU-620 HX Series 620W - $302 -- $169.99 USD
Silverstone Zeus 75ZF (etasis) - $301 -- $199.99 USD
Enermax Liberty 620W - $281 -- $169.99 USD
Silverstone Olympia 650W - $259 -- $149.99 USD

Interesting that the Corsair is the same price as the Liberty in the US, and the Zeus is cheaper here.
I'll drop the Zippy, on account of it's price tag. But anyway, why do you recommend the Olympia?

edit: What dimensions define a small case?

Ok, I read a review by Jonnyguru; the olympia is good. So are the Liberty and the Corsair, mind you. The Zeus is very grunty but I don't like it. In terms of future safe, the Olympia is the best, but I do like the features of the other two.

OK.
I can now get an Enermax Liberty 620for $260.05 NZD, or a Silverstone Olympia 650 for $259.00 NZD. (Or a Corsair HX 620 for $302 NZD, higher price but 5 year warranty). I think the Corsair is too expensive compared to the other two, for similar performance. Anyway, the Liberty is modular, with slightly less wattage, whereas the Olympia isn't. However, jonnyguru has rated the liberty as a tier3, vs corsair and olympia as tier 2, because he has heard reports of several failing in aciotn. It has a one year warranty against the olympia's 3. Olympia = 54A on the 12v rail. Liberty is 22A on 12v1, and another 22A on 12v2. Does anyone know if the combined total is 36A? I heard it is but I don't know... If this is correct I will buy an Olympia so please le me know NOW!

Ta. And I was correct. 36A vs 54A.