At this point, a 7200 RPM drive will be a modest improvement, but your best bet would to either get a hybrid drive or go full SSD (that's the best option). I had a friend that was ready to sell a machine with exactly the same configuration as yours because it was getting too slow, but I talked her into getting an SSD (Samsung 840...this was just before the 840 Evo was introduced), and it felt faster than my then brand-new MacBook Pro (mid-2012 13", 2.9 i7, 8GB RAM, 750GB hard drive) for most tasks...that changed when I installed an SSD myself, but her computer still feels very fast for an almost five-year-old machine.
If you're going to spend some money on a machine like that, keep an eye out for deals and just get an SSD - you can always put a hard drive in the computer's optical bay or use it externally for additional storage of non-critical items. The hybrid drives certainly are nice, but they still are a bit of a compromise until the cost-per-GB on SSD drops.