I7-4700 and Geforce 750m sli- how good will league run?

TL;DR GPUs are hot garbage, CPU is fine.

SLIing 750ms is incredibly dumb on just about every level. It is inefficient, and doing it with super-cheap cards instead of just buying a twice-as-powerful cards is a really bad idea. This is why only the most expensive cards even have SLI capabilities, now.

LoL would run at max graphics/60fps on just about anything, a single 750Ti would be plenty, but the 750M is the lame laptop version and to my knowledge is based on a previous architecture (750Ti is maxwell-1, 9xx series is maxwell-2).

Edit: looked it up. The 750Ti is twice as fast as the 750M, even more in some cases. More than i was expecting. A modern 100$ GPU (bare minimum for general 1080p/60fps gaming) would be around four times as fast.

If you are looking for a new computer, i can give some suggestions for a prebuilt or for building your own (cheaper, upgradable, quality assured). Just list the software/requirements you are looking for.