Egon is right; thanks for pointing that out. I boofed. Sorry about that.
I was ignoring the "suction head" aspect of the closed loop which should be subtracted from the discharge head, essentially canceling one another out.
So with the head pressure from the Filter (on average 5' of head) and the friction head added by the plumbing the total head is going to be around 6' (assuming you plumb the tank with ~10' total of 2" PVC and have 2 or fewer 90 elbows).
Here's a handy calculator that might help if you want to double check my figures: http://www.aquaticeco.com/calculators/pump-calculator.htm
So depending on which Flow chart you use for the Reeflo Dart (I came across three of them), you would either be getting 3,010GPH, 2,750 or 2,428GPH @ 6' of head.
The Dart will work just fine for the Ultima 4000. Sorry for the misinformation.
In regards to the Ultima 2000, the Dart may be too powerful for that model. Aqua UV recommends a max flow of 2,500GPH for that model, so your pumps (depending on what flow chart you look at) may be too powerful.
I'm not sure if noise is a factor, but in my experience, Hammerheads have a pretty loud hum, whereas my Super Dart Golds were very quiet. The Hammerhead does ~5,000GPH at 6' of head, so according to Aqua UV, that would exceed the max recommended flow for the Ultima II 4000, so you should jump up to the 6000 model.
In case anyone is interested, the best write-up I was able to find in regards to head calculations is here: http://www.mcnallyinstitute.com/07-html/7-01.html
