The i7 and i5 are two sub - series in Intel's newest processor series. As the numbers indicate, the i7 > the i5, but the convoluted nature of intel's current processor lineup and socket options make it difficult to give a concise explanation of the differences. By graphics processing you mean photoshop stuff, right? If so, go to the charts tab on the top of this page, select the processors tab in the sub menu and compare the processors that you are interested in in the photoshop section of the 2009 test charts.
i7 and i5 are marking the "category" of the models
The main difference between Core i5 750 is that all Core i7 CPU's have Hyper-Threading: It has 4 cores, but each core is acting like 2 threads, rather than one, so each core slips the part of the process that comes through it into 2 smaller processes, and when all is processed, then is returning it back as one part of the process. In this way its getting the job done faster and the OS is seeing it as an 8 core CPU
The other difference is that Core i7 platform (X58 chipset) isn't limited to using x8/x8 mode when doing SLI or Crossfire. Core i5 (P55) are limited to having the full bandwith (x16) when only using one PCI Express 2.0 slot, and x8/x8 when using two GPU's. X58 also enables the full x16/x16 when using two GPU's.
Also, Core i7 uses triple-channel memory, rather than dual channel (Core i5). Triple-channel increases performance, but not by a big amount.
In games these won't do much of a difference (except the Core i5 6xx models, which are dual-cores, but also have Hyper-Threading, so the system is seeing it as 4 core CPU, but it isn't a true Quad-core)
If you consider a Core i5, get the 750. It doesn't have Hyper-Threading, but it's the best value CPU from Intel at $199.
If you want extra-performance and system that won't need an upgrade for several years, get the Core i7. But get either the Core i7 920 or 930, the best value Core i7 CPU's