At this point, I'd probably recommend holding off on your upgrades until the new Intel processors (Sandy Bridge) become available early next year. When they appear, sell your CPU/MB/RAM and buy a new Sandy Bridge core system.
Can't really futureproof a 775 platform system... Stick with what you have, or part it out and build new after the release of SB.
CPU - The dual core @ 3.75GHz is respectable. If you upgrade your 775 platform to a Q9000 series CPU you will be spending a good ammount that could go toward a new build. If you really want a quad core look at the Q8000 series for less $.
Buying an SSD will benefit you now and in a future build. Look for an SSD with a SandForce controller (OCZ Agility 2 is one example).
Upgrading from 4GB memory to 8GB would be nice, but buying DDR2 will ONLY benefit you in that system. When you upgrade the MB you will be using DDR3. Not sure the cost of the additional DDR2 4GB is worth it unless you are keeping that rig for a while...