Hello, i have an older comp. and i want to do as many upgrades as possible?

my comp. is a compaq preserio pc sr5010nx, it is 5 years old and i know that in comp. years that makes it a dino. it runs an intel celeron D 360. the motherboard is a 945gct-hm (livermore). i have it maxed out at 2gb ram, now the question i have is, i am on a limited budget and can afford only small changes at a time, is there a way i can upgrade this comp. for speed and ram without breaking the bank. or do i just save my pennies and buy a whole new system. thanks