I bought my wife a 11" Acer netbook with 2GB RAM, the AMD C60 1.0ghz cpu, and a 320GB 7200rpm hdd. It was so super slow it was almost unusable. I upgraded the RAM to 4GB and swapped out the hard drive for a 180GB OCZ Agility ssd. These two upgrades (combined with the clean install of Win7) turned that netbook into a pretty damn quick little laptop. Battery life went from about 5.5hr to about 9hr too.
I have a 17" Dell laptop with an 3rd gen i7, 16GB 1600mhz RAM, and a 1TB 7200rpm hdd. Doesn't sound like I needed an upgrade right? Well, I moved that 1TB hdd over to the 2nd slot, and put in a 128GB Samsung 830 ssd into the first slot, did a clean install of Win7 and it made a huge difference. Everything is faster.
I have a 10-11year old Gateway laptop that is still usable. It came with 1GB RAM, a single core 2.6ghz AMD Athlon 64 4000+, and a 4000rpm hdd. I maxed out the RAM to 1.5GB, swapped the hard drive for a 7200rpm hard drive, and did a clean install of WinXP. It made a huge difference. It's reasonable quick for such an old laptop.
Years ago, I had an old Dell Dimension E521 desktop that came with an AMD Athlon 64 X2 Dual-Core 3600+ 1.9 GHz. I was gifted a desktop for parts that had a AMD Athlon 64 X2 Dual-Core 5400+ 2.8 GHz. I swapped the cpus and even added another fan to the case. It was decently faster, but it didn't make as big of a difference than I expected. It was a free upgrade, but I think it would have probably been worth about $50. But that is a 900mhz x2 upgrade. You will notice a small improvement in such a large clock speed difference. Even still, it did not compare to a ssd upgrade. The hard drive is almost always the bottleneck in any system.
My cousin had an i3-2100 in his desktop and I put in an i5-3470 for him, and he was not happy with the performance increase. He sold that rig, and had me build him an all new system with an A10-5800k. I insisted he get a ssd for windows from the start so he took my word for it. Not only is he happy with his system, he swears it's faster than his i5 setup was. We all know an A10-5800k is NOT faster than an i5-3470.
If you can find your dad a better Core 2 that is at least 700-800mhz faster than his current 2.2ghz, but doesn't cost like $100, get it. 300-400mhz faster is not going to make very much difference, and may not even be noticeable. Even still I don't think I would pay more than about $50 to upgrade a cpu on such an old platform. Especially considering you can buy a brand new Intel g1610 + motherboard for about $80 that would destroy his current system.
Windows can only operate as fast as your hard drive will let it. With a ssd, windows can run at 100%. Everything is instant. Booting, starting, loading, installing, scanning, saving, searching, sorting, opening, closing, hibernation, sleep, and data transfer speeds are all considerably faster. Literally everything windows does is faster. It was even noticeable going from 4000rpm to 7200rpm. The only thing a ssd won't do is make a game play better.