We need to know your motherboard, or manufacturer, to advise you fully... and your hard drive.
Usually you can upgrade the video grapics card, hard drive size and speed, memory amount as well as increasing speed a bit. Then with older machines it is always wise to replace the parts that have a lot of wear: Power supply, cpu fan, hard drive and optical drives... which, of course, you can do component by component on a budgeted approach.
Remove the heat sink, to clean and replace the thermal paste.
Check for updates to your software and security system.
Perhaps a new monitor and graphics card combination.
External backup drive.
We can always think of ways to spend somebody else's money, but also avoid the disappointments of unplanned crashes.
I would start with a bigger, better, faster hard drive, and cpu fan, then a more powerful, quieter power supply, then an optical drive... assuming the memory is maxed out.