First of all, it is not a big problem to overclock e5450@4Ghz if you've got a good mb based on p35 or p45 chipset, second one is better of course. If you already have a lga775 mb, then you only need to buy good cheap cooler, for example Zalman CNPS10 Optima or similar. Then check you PSU, it must give "true" 450-500W. Kingston kvr 6400 will be good as ddr2 RAM to overclock. The problem of 2 core cpu's, unlike 4-core, is that they will give short freezes in games, as gta5,witcher3, optimized to multicore systems. It is very uncomfortable to play. But you'll have a little bit more fps in 1-2 cores based games, such as WoT without any problems. But truely, I don't have any problems with any games even in 2560x1440. My system: Xeon e5450@4 Ghz, Asus p5q-pro, 8Gb RAM, GTX970 4Gb.
For example: h**ps://***.youtube.com/watch?v=ZbglcO1QFjM