Hi,
I'm guessing you're running Windows XP (based on the spec. you've given). 256Mb ram is just about enough to run the operating system (without anti-virus, firewall, browser etc.). A ram upgrade is definitely recommended. 512Mb will help, but I'd recommend going for 1Gb for future proofing (and it'll really make XP hum). First... go to
http://www.crucial.com/ and input your make and model etc. You'll get comprehensive information on the type of ram you need. Spend a good bit of time reading ALL the info... the max your system can take, whether it needs to be paired etc. Crucial are not cheap so, armed with the info, go to Ebay and enter your specifications for a saving of approx. 50%.
*** Warning *** AVOID high density ram. This will only work in approx. 10% of motherboards. It will either not work at all, or will only work at 50% capacity. Ensure it is LOW density. If in doubt, contact the seller through Ebay's messaging system to ensure communications are recorded.
This is how I buy ram at the best prices... hope it helps.
Growing old is mandatory... Growing up is optional.