Hi Latrobe,
I looked up your mobo specs & outside of the small & really non-issue of the PCI-E lanes being PCI-E 2.x spec (since you're using a single vid card), you got a good setup.
As also mentioned you got a good CPU that can OC very well (if OC'ing is something you would consider) & get some extra performance there w/o a CPU upgrade. Your socket is an 1156 socket which allows you to a wide range of CPU upgrade possibilities but when I checked the mobo BIOS's available for your mobo on MSI's website none gave any hints at any issues between a SB, IB or Haswell vers Intel socket 1156 I3, I5 or I7 CPU working so this may be a trial & see endeavor so you may want to weigh this out if your really considering a CPU upgrade (I'm assuming that your current CPU is a SB CPU). Currently I couldn't find any listed roadblocks that would limit your choices here so the sky looks to be the limit.................
The vid card is a place where you could gain some creditable performance & IMHO the GTX 970 vid card is the 1 to use on this platform (as already been mentioned & assuming from the current GTX 760 card you may prefer Nvidia). If this were my platform I would do this...................
The 1 other area that I see but not enough info is given that some more performance can be squeeked from is your mem modules.....not talking bout their capacity but their frequency or speed. Your mobo specs out to accept mem frequencies up to 2133Hz. You didn't give what the current mem frequency of your mem is just the capacity. If you were to check the white papers on the I5 2500K CPU the mem speed specs are listed at 1066/1333 but from my experience they can usually work with mem approx 267Hz faster than the max spec....over this I don't know. I haven't found anything that says that they won't work with much faster mem so the limit should be what your mobo/chipset can support which is 2133Hz.
Now the mobo specs will say that this speed is attainable w/ OC. That was the case at the time of manufacture as there were no DC sets of matched 2133Hz mem kits available so you had to OC the 1333/1600/1866Hz sets to get there. This is not the case anymore as the matched DC 2133Hz mem kits are available so the BIOS should read the mem SPD & use it at the rated speed as long as the mobo supports the speed range.
In all you got a good set up that should run most apps/games very well but if you want to upgrade it then I say go for it as you can do some things w/ the mobo/CPU combo you have already.
Hope this helps.................