Quote from Scataphagos:
For a multi-monitor system, you need to have proper proprieties..
#1. How many monitors to run (and allow for future expansion)?
#2. Which video cards? x16s, only... or x16s + x1s.
#3. Which mobo to accommodate those cards?
#4. Which computer has the mobo you need?
Unwise to "buy computer first"... then assume you can get all the monitors you want to run on it.
You've wondered "whether a T3500 can run 8 monitors"? As you have a T3500, you can test it for yourself. (a) Run 2, quad cards, or (b) 2, x16 dualheads + 2, x1 dualheads... as you'd find in the Nvidia Quadro NVS 295s and 450s. Of course if you buy a couple of x1 cards for the test, you'll likely feel compelled to use them.
If you want to run "dualheads on all x16s", then you can't use a Dell.