Computer help: RAM cards vs. Virtual Memory

RAM cards vs. Virtual Memory

I thank you in advance for any information/assistance with a question. I have a Compaq Presario, Model 5822, SN 1x99ctq604cb, and I run XP Home Edition, 256 Mb RAM, no special programs, CPU speed 499 hz. I recently had a LOT of BSODs and I installed MemTest which showed I had a bad 128 Mb card. I replaced it with a new one and now I am up to date RAMwise. Things were fine but lately it is VERY slow. I am a regular user of the Glary Registry Cleaner and use that program to clean, defrag, optimize and just about everything else. The PC was built in 1999 I think. My question is, I think I need more RAM. Can I either put in a 256 to replace one of the 128s or do I need to have (2) 256s which would take me to 512 RAM. However, is there anyway to tell if my motherboard would be able to handle 512 since the PC was created to use 256. I have deleted all unnecessary items from startup, removed RAM users from the task tray and still, from time to time get the warning that Windows is increasing memory because I am low on virtual memory. Oh, I also have a 10 gig HD and I have between 3 or 4 gigs free. I have noticed that when I hit control/alt/del and pull up task manager, the tabs are no longer on the window. I do not know if this is related or not but it is a new thing that is going on. I can re-run the XP CD if necessary but do not know if that would help the task manager thing. If anymore info. is needed, please let me know and I will be glad to furnish it. Thank you again for any information or assistance.
Tony