if I go big wattage do I have more cables
While it does depend on the PSU, the wattage also plays a part. Assuming we are talking about fixed cable PSUs and not modular ones, a higher wattage PSU of equal quality will have extra cables. A 750 will have more molex and PCIe cables than a 450W will. It's been awhile since I looked, but I think my 450W has the 24pin PCIe bundle, the 4+4 cable, A single PCIe cable with the two sets of 6+2 pin plugs on it, and then I think a total of 4 strands of SATA/molex plugs. A 750W will have more. Another strand of PCIe plugs, and more SATA/Molex plugs. Modular changes that of course as you don't have to have them plugged in.
And since I only have a small gpu I feel getting a 750 would give me the option sometime to put a much better card in there and maybe watercooling, obviously not until I buy a better cpu.
I thought that way once. Bought a 750W because I thought I was going CF/SLI. Here's the problem with your thought. First, you need to buy the new CPU. And the new cooling equipment. And the larger GPU. And even then you are looking at something that can be powered by a 550W PSU. That's a lot of money spent on upgrades and you still don't need the 750W PSU. Worse, if this takes you a lot of time, how will PSUs be changed by then? My first good quality PSU didn't have 6+2/8pin plugs on it because they didn't exist when that PSU was made. I had to go out and buy a new PSU to support my R9 280. By the time you go out and buy all this stuff (CPU, cooler, higher wattage GPU, etc.) what new things will your PSU need to support? I haven't even started talking about efficiency yet. 450-500W is really the sweet spot for a PSU.
So am I correct that my current mobo only supports 1866 then without clocking. Or can I still put the higher mhz ones in
The board I'm sure supports more. Because the memory controller is in the CPU that matters as well. You should be able to put 2400MHz RAM in there, but it wont' run at 2400MHz. Unless you are OCing and need the higher rated ram it isn't worth the expense.