My two cents:
The R7-250 is a fairly low-end graphics card so it would be a good idea to know what you are replacing. If you don't even need gaming graphics there are cheaper cards. You also have an iGPU but I wouldn't use that as it would use up some of your System RAM.
(If you have 32-bit Windows AND don't do any gaming then your CPU's iGPU might make sense)
Pricing and performance:
R7-250 starts at $70 (comparable to HD7750)
R7-260 starts at $83
EVGA GTX750Ti is $110
http://pcpartpicker.com/part/evga-video-card-02gp43757kr
http://www.techpowerup.com/reviews/ASUS/GTX_750_Ti_OC/25.html
The stock GTX750Ti is exactly DOUBLE the performance of the R7-250 assuming no CPU bottlenecks, so in reality with your CPU and the better EVGA GTX750Ti's overclocking capability some of your games will be slightly CPU bottlenecked but some will go higher than 2x the R7-250 so let's just roughly call it DOUBLE the performance on average.
The GTX750Ti is also a 2GB card which is a bonus but ONLY consider if you have 64-bit Windows otherwise stick to 1GB video cards.
*If you're on a strict budget, then consider the R7-260 1GB card for $83 which comes with TWO FREE GAMES as well:
http://www.newegg.com/Product/Product.aspx?Item=N82E16814127780&nm_mc=AFC-C8Junction&cm_mmc=AFC-C8Junction-_-na-_-na-_-na&cm_sp=&AID=10446076&PID=3938566&SID=
That's only $13 more than the cheapest R7-250 through pcpartpicker for a noticeable improvement. Not as good as the GTX750Ti but the reduced price and games makes it an awesome value.
Summary:
1) I recommend the MSI R7-260 (+ 2 free games) for the best value. I can't recommend the R7-250 for about $13 difference.
2) I recommend the EVGA GTX750Ti for best performance for a reasonable price.
3) 1GB for 32-bit and no more. Preferably 2 GB if you have 64-bit Windows and budget allows.