You do realize runescape runs off of your CPU and doesnt use your GPU at all right? The only thing OSRS uses GPU wise is OpenGL mode on OSBuddy so if he's botting he doesnt need a GPU. He should focus on good RAM and a good processor instead of wasting money on a GPU.
Since RS is a single-core usage game (many games haven't adapted multi-core usage yet) the i5 is definitely worth the extra money.
@OP
Do you want a gaming pc, or a pc specifically tailored to botting? Feel free to add me on skype and ill make either for you.