With that hardware you'll want something above 600 watts, the more the better. You'll want a cpu lead of 22inches or more so you can route the cable in a desirable fashion. I HIGHLY recommend modular if you're getting the 900, its a great case, but wiring is a PITA. I even cut holes in the backplate and it still doesnt look that clean (the holes are too small and my xfi header ide cable wont fit in my holes).
Silverstone Decathlon is an excellent PSU and is fully modular. Is your budget for canada or USD, I found a Silverstone Decathlon DA650 for 160.06CAD. I would lean towards a 700 watt psu myself, but 650 will cover what you are putting in there with room to spare.
Another factor to consider is having 8-pin PCIe for future graphics cards. Enermax Infinity (note the CPU lead isnt all that long) has this, as does the galaxy series and Silverstone Decathlon. Thermaltake Toughpower features the 8-pin PCIe in some select models as well.
If you want an excellent PSU that will last you longer than your current rig I would get one of the following.
1. Top choice->Silverstone Decathlon<-Top choice
Features 8-pin PCIe, rock solid stability, fully modular, and quiet
2. Enermax Galaxy/Infinity
Best in class stability, galaxy uses rubycon capacitators(best of the best)
3. Thermaltake Toughpower
Ranks right up there with the galaxy in stability, highest efficiency of all
The Gamextreem isnt that good at all. The proXtream is actually quite good. Of the two you suggested the Thermaltake is far and away the better choice, its an excellent PSU. So yes it is worth the extra 73.00. Keep in mind though the silverstone decathlon's if you have a case like the 900, you'll get the same if not better performance from it and have the modular design to work in your favor.