i don't think you would be able to if you go past whatever length newegg will cover the parts for. not sure how long that usually is, but it should be easy to look up.
really, though, trust the manual over newegg. the odds of a new-ish motherboard only supporting 2gb of ram are tiny.
and, vivify, you need memory so, so badly. it's pretty cheap now, so you should be able to find a good deal. if that machine had windows 2000 on it, it may take plain sdram(when did ddr come out? i can't remember). newegg still has some in stock.
about the only thing you can do for the time being is to up the size of the paging file(virtual memory) and make it a static number. for 256mb of ram, making the virtual mem 512mb(set init and max to 512) should do. it won't make anything a LOT faster, but it may help a bit.

as for anti-virus, avast! is better than avg and is free also. i should really get around to putting that on this computer and get rid of avg...
also, for malware, malwarebytes' anti-malware and spybot s&d are nice. spybot is nice for it's HOSTS file protection and malwarebytes' anti-malware is just really good in general at detecting and removing most anything. i'm not much of an adaware fan, considering that the above mentioned programs will handle most everything it could.