That's a hard decision! First off, both GPUs are good value for money. They're both going to give you a big increase in performance over your GeForce GT 240. The biggest difference between them is going to be the price.
The Sapphire HD 7770 GHz Edition sells on Newegg for $104.99 while the MSI GeForce GTX 650 Ti BOOST is priced at $154.99. This is nearly 50% more expensive, but does it provide 50% (or so) more performance? No. It provides around 10-15% more performance on average over the HD 7770.
Our own GPU Editor Shawn Baker took a look at the MSI GeForce GTX 650 Ti BOOST GPU, with some of his benchmark charts comparing the HD 7770 to it. I would suggest checking out his review, which might swing your decision.
My advice, would be to get the HD 7770 as it's $50 cheaper and provides nearly the same experience at lower resolutions (which your 18.5-inch LCD would be). If you were talking about 2560x1600 (or so) then I would suggest buying a completely different GPU altogether.