Will a GTX 750 Ti fit my motherboard's PCIe slot?

Yes the 750ti will work no problems, it's not if it will fit the motherboard it's just a question if it will fit inside the case you have plenty of room.
Everything should work.
If you haven't bought the power supply yet I would consider buying a different brand.
About 8 bucks more at Newegg.
http://www.newegg.com/Product/Product.aspx?Item=N82E16817151094

yes
it will fit any grafics card as long as the case you bought is bigh enough
but the 750 will most def fit
i would get this if i were you
like 20$ more and twice the power nearly, pluse DX12 full support do future proof. will fit as well.
http://www.newegg.com/Product/Product.aspx?Item=N82E16814125809&cm_re=950-_-14-125-809-_-Product
edit just checked his case it has plenty of room to acomidate alomoast any graficks card size. i would not go out and get a titain though.

i3+mobo+2stick ram+129 (149w for 950)watts of power for a 750ti? he does not need a new power suplly .

Zerk is right about the Power Supply. It is a low quality Tier 4 on the Tom's Hardware PSU Tier List 2.0. It is not about the wattage in this case, it is about the quality of the components.

im not saying he is "wrong" im sying the guy want to buy a grafic card and if he was running a i7 with an asus vIII then yes he would need a new PSU.
but he is on a budget trying to get an office pc to play some games and if you take his buget and take 50-70 out of it when a corsair will handle a new 750 untill he gets more dough then he should be fine, and in my pespective i belive i have seen pc run on a lot worse than a corsair. and have suvived fine.
would you rather? psu
and crap GFX
would you rather uptodate GFX on pare with PS4 and likes
and wait it out
and by an decnt PSU when you ALso you /need the required stbility / power
i am not dicrediting him im saying not to take away from the guys budget! or are you handing him extra to go buy a decnt PSU so his future 750 wont over load it. not everyone rolls in money, has a decnet job to where 2-3 pc in the famaly is the norm, or has parent that will buy them all there great things they wish for all the the time just cause it not "tier 4" . grow up and thionk out side the box.
hey i might be wrong he/she could be a milionair.. but then wtfranken wieners is he/she trying to play games on an i3 for?!

no you will not have to change a thing, once you put the card in just plug the power (if it has any)in the card on the side, (some 750 can use the pcie to get power) and plug the monitor into youcard and install drivers thats all happens almost by istelf plug and play easy.
you will basicly have a steam mashine when your done, good luck. have fun!

That was a suggestion to get a higher quality power supply for a very similar price, in general for a low watt PC a CX is OK but you must look at the future for upgrades also.
Not a clue why you would think the 750ti does not support DX12.
http://wccftech.com/microsofts-directx-12-api-supports-amd-gcn-nvidia-fermi-kepler-maxwell-intels-iris-graphics-cores/

All of this was funny when you suggested buying a GPU that cost more. The 8 bucks would of been a very good investment for the future.

it support directx 12, with win10 as do all, but only from the "driver" side, its hardware does not fully support direct x12 cause it was originally meant only for 11x good job nvidia/microsoft for finding a driver workaround so the people that did buy previous gen cards would not get left in the dust.
the 950 however does support direct x 12 from the get go to keep up with amd/ATI and that is also why i suggested it, being that it only cost 20 more than the card he chose.
adding 80$ dollars (not 8) to in additional to the 120~$ (total 200$ ) card would theoretically be a good investment for the future, but as i already said before a budget build that he A)already has the components for becomes overpriced because now he has an extra PSU laying around and B)is out of his buget cause if he could afford a 200$ card he would prolly ask for something in that price range.but again "buget build" C) it WILL run fine with his build and the card he chose, not the future card , not quantum computer the NSA has but his : i3, 2ram, 750ti, HDD, CD, mobo, fan that he has NOW not later. that is NOT what he asked.
and more importantly just cause one person agreed with you for a brief second doesn't make it true, make me come school you because you seem to have the need to have the last word, and you even got that wrong, when the guy is happy and the problem is already solved! so, go talk it over with you girlfriend, wife, teacher, mothersisterbrother.. who prolly all hate you just as much as i do right now because you are such an arrogant ass, and if none of those kind of understanding people are available to you then google or read something! because every once in awhile i'm not saying ALL the time, but sometimes you are wrong.
and you youeelf have a corsair TX 850 has is that any difrent from what he has ? you should
http://www.evga.com/Products/Product.aspx?pn=120-G2-1000-XR
they are excelent quality! far superior than than what you have and very futute proof, so don't sit in a glass house and through stones, hope you corsaire fris all you shit tmw just for taking up my time to put you in your place and bealite more professional nextime.