wimpy100 :
I'm looking to upgrade my Radeon hd 4300/4500. Elsewhere on this forum there has been a suggestion of a Nvidia GTX 750. I've also looked at the Radeon R7 250, which is about half the price of the GTX 750 TI. Is the 750 TI really twice as good as the R7 250?
I would be grateful for any help or advice
Usually, no. (For example the Titan X (Pascal) is about twice the price of a GTX 1080, but only about 30% more powerful) But it all depends what you're upgrading from and to. In the case of those two, the 750 Ti is almost twice the power of the R7 250 2GB, actually about 90% more power. But take into account the 1050 that just came out and the difference is more like 200%, or three times as fast.
So depending where you are you could get a 1050 for the same price as a 750 Ti, and get much more power.
But if you cannot get a 1050, especially since they are a bit price gouged right now, the 750 Ti is just barely less than twice as powerful as the R7 250 (assuming that you meant the 2GB version)
But if you are upgrading from Radeon HD 4500 to even just the R7 250, thats going to be a MASSIVE increase already. The difference will only be greater the higher up you go, either a 750 Ti or a 1050. Both are about the same price in the US.
R7 250 2GB ~ $60 (about 700% more powerful than the HD 4500)
GTX 750 Ti ~ $100 (1400% more than 4500, 90% more than R7)
GTX 1050 ~ $120 (2100% over 4500, 190% over R7, 50% over 750 Ti)
and for good measure, the #1 rated "budget" card in the US and UK;
GTX 1060 3GB~ $200 (4300% over 4500, 475% over R7, 200% over 750 Ti, and 100% over 1050)
But it will bottleneck unless you have a good CPU.