That's a pain. The motherboard does apparently support 8GB RAM so you are probably looking at getting 4x 2GB sticks.
Though, did you try reseating the memory or just one stick at a time to see if that would post?
In regards to upgrades, I don't really feel it is worthwhile either overclocking or upgrading the CPU. Overclocking does add strain to the components so if you want it to last longer, I personally would not bother. If of course you just want it to tick over for a couple more years and want improvements in the mean time, feel free.
Another option would be upgrading to a Q9xxx CPU would give some reasonable improvements and use less power. They should worth fine with a p35 based board, but best to double-check with the board's documentation if you decide to go this route. Again, not sure it is worth the effort unless you can get one extremely cheaply.
SSD is a very worthy upgrade and a 480GB one should still be useful for a potential new system further down the line.