Hi,
Page #9 of the following document shows RAM you can install on your machine (max):
http://h18000.www1.hp.com/products/quickspecs/11814_div/11814_div.PDF
Finding drivers for Windows 7 could be harder than upgrading RAM. Please check the following post:
It's a good machine isn't it. I had one and loved it (not using now) that why I'm here. I also had one nx8220 machine. The nx and nw series were good machines and very expensive at that time (9 or 10 years ago).
Regards.