Here's my two cents (extending from certain comments while overlooking others because of their lucid and sound advice):
For one, if you're not partial to any case then you may want to invest in a Lian-Li PC75 case. There's room at the top of the case for a 3x120mm radiator but it will require some modding (a hole cutter and things of that nature). The advantage to this case, in my opinion, is that you don't sacrifice room for other things (internal/external drive bays, pump room on the floor, etc)
Next: a resevior is almost certainly unnecessary! Someone over at XS tested his flow with a radiator infront of the pump inlet and without one, and he saw some minor differences in flow. The only real advantage to using a radiator is when you go to fill the system, you just turn the pump on and if there's enough water in the resevior everything will fill up quickly in one shot but if you use a T-Line then you must jump-start your psu and shut it down before the water in the T-Line runs out, which is usally a 12-18" long piece of tubing 1/2" wide. Note: if you run your pump without water it will drastically reduce the life of your pump, if not destroy it, therefore, you must shut that pump down in the T-Line before the water runs dry.
Not to mention: if you are going to run seperate loops and you decide to use a resevior then you're going to need two reseviors because most reseviors only have a single inlet and a single outlet (some have three so that you can add water to the resevior via a T-Line). The same goes for radiators! Now, in theory, you could use a couple "Y" connectors and converge two loops into one and pass the single tube into the radiator inlet and then diverge the single outlet path into the two original loops but this will inhibit flow to some degree and with highly impingent waterblocks, high flow is imperitive for achieving the results you expect from these high priced products.
Also, by the looks of your diagram, I see you're leaning towards the NV78 block (assuming that's the SKU/Part number of the picture I'm looking at). To eliminate any confusion, the NV78 cools both the GPU and VideoCard RAM. You would see much better flow if you were to use a Maze4 (brass cover, copper base) and put some BFG or OCZ ramsinks on the RAM. If you don't want to take my word for it then look through some of MaxxxRacers posts at XS and his posts will corroborate everything I'm saying, seeing that he's got data to back up his suggestions (in case you are wondering, he's about as knowledgeable as they come in watercooling). You'll save some money buying 2 x Maze4's and however many ramsinks your videocard requires as opposed to buying a NV78 comparable waterblock, plus, you'll benifit from better flow which will improve cooling as a result.
Also, the AX 50z's are great pumps, no one can dispute that. However, something to keep in mind is, the highest ranking man involved in Swiftech Watercooling (Bill M) had the 50Z's recalled because of a high failure rate. (swiftech had a different part number but that's irrelevant). Maxxxracer is investigating these allegations (or has investigated, it's been a while since i checked back in on that thread) to determine whether or not he should refrain from recommending them anymore. So that should pose as a caveat.
The Iwaki pumps come highly recommended by MaxxxRacer. He loves them! But you should really read up on those pumps before you buy them because certain models cost a lot of money plus some dump more heat then others will and only yield a small degree of improved flow. There are some other things to take in account when buying those pumps, like good tubing (the pump is so strong it can actually suck the tubing towards the inlet shut). Read up on it, I can't afford the pump so I only know the little that I've read on it.
Also, make sure you keep flow in mind at all times! The sharper your bends the more flow is likely to decrease (sharp bends should only have a negligable decrease in flow) but elbow connectors and "Y" connectors will slow flow down, particullary elbow connectors (aka 90 degree connectors).
I'm tired of typing so if you have any more questions then just ask but the best thing you can do is read read read!
RollerCam540
Also, another supposed benefit of the reservoir is that you can bleed the system of air in a matter of minutes as opposed to several days for a T-line.
