Might just leave his system alone if it's not going to make a difference and just get my sister the same cpu or something. I would just like to get her something faster then what she has. My father mostly uses his computer for the internet, word processing, youtube, etc. Has a ton of programs loaded and always has multiple programs and windows open. He's and still running xp with 2gb ram. I don't think he needs a quad, a dual core or something is fine. He does like speed though and seems to think the faster he gets into windows the faster his computer is.
Would overclocking the chip help all that much?
Who's got which chip now? Sister has the 5050e? That's real old tech. The Athlon X2 250 would probably be perfect for each of them if all they do is surf, email, music, word process. If they do more complex operations, you might need more oomph.
Your dad's rig would benefit from more RAM, Windows 7, and an SSD (set up strictly as a boot drive, this takes some time, but would reduce his load time to seconds. SSDs don't love WinXP though, and you'll again have some work to do). The RAM and the SSD would really improve muliti-tasking. How much is in your father's system?
As to overclocking: it will improve performance, yes, but if the rest of your system is bottlenecked while multi-tasking, it will not do exactly what you want. People on this community range from gamers looking to push out more FPS via OC or hard core overclockers looking to set world frequency records. So we have a bias toward overclocking even when it isn't the appropriate answer.
Anyway, if you're really itching to upgrade, Intel has a highly underrated value oriented chip in the i3 530, and AMD's Athlon II x2 or x3 line offers tons of processing power for the average joe at bargain bin prices, and likely without the need to replace Dad's motherboard. Rebuilding a system from scratch with today's modern parts will increase performance overall. But it will cost into the mid $200s, at least.