further 256 should only cost about £50.
Blimey that's dear! I just brought another 1GB stick that was about £75 from crucial, now have 2GB's doesn't really decrease start up times, increases game loading and processing online game, used to start a game and be last in, increasing RAM offloaded the swapfile/virtual memory being loaded on the HDD after that installed a new HDD, that speeded things up a little.
Do regular av and spyware cleanups, defragment HDD once a month. The biggest hog is things being loaded at start up, reduce that and it should be quicker.
By using msconfig you can stop programs being loaded at initial startup which may help the start up process being quicker.