There are three basic "upgrades":

• faster processor
• larger, faster HD
• more RAM

I believe the faster processor, if available to you, is going to yield the biggest bang/dollar spent. OTOH, RAM is cheap and you may find a slight benefit. Some folks are fitting much faster HDs + larger capacity. That can also give a minor improvement. Most of these components are cheap and the market is flooded so it's not costly to experiment assuming you can do the retrofits yourself.