Whoa whoa... yes, you certainly can change it, but not with XMP on. Your whole system is regulated by the base clock. Stock is usually at 133. Your CPU has a multiplier - at stock it's "20" but in reality it's 21 on 4 cores up to 24 on 1 core (this is turbo boost). The QPI runs at 34 or 36x multiplier. The RAM runs at 6, 8, or 10x. XMP simply set your baseclock to 160, so the RAM is running at 10x. You could, for example, set 200 base clock and the RAM to 8x for 1600, and then choose a CPU multiplier for, say, 3.4, 3.6... 4, 4.2ghz.
I currently have an i5 750 set to 178 base clock. Turbo is on, so it runs 3.75ghz up to 4.3ghz with turbo. My RAM is also 1600mhz RAM (CL8) which I have running at 1426mhz CL7. It took a tad extra voltage to get it stable at this speed, but that's because the timing is faster and overall the RAM is performing better than 1600mhz CL8.
There's a lot of guides on i5 750 OCing, I suggest you start googling and having a read. There's actually a great one on this forum but I can't find the link right now... used to be a sticky...
No the absolute highest multiplier on this core you can set is 21, if you have EIST on otherwise it's 20. Only the "k" versions of Intel cores have unlocked multipliers, so for example the i7 875K you could set 160 x 25 even, maybe more I don't know. But you pay a lot more for that "k" as well. IMO just try enabling turbo boost and you'll gain some good performance, it'll boost up to 3840mhz on single threads.