hehe...256....that's a given for any system these days. It kills me to see these bargain boxes---and don't EVEN get me started on the laptops with only 128 in them.
Any NT kernel'ed system should have a MINIMUM 384 installed--512 for smooth sailing. Until you cross the 300MB threshold, it will eat up almost 2/3 of physical RAM just loading the OS and Network services. If this falls below 170MB, it starts to swap out to disk pretty quick. At 66% load, this computes to roughly a 256MB install just to get it to run decently. You'll need more if you intend to do any developing, video/photo/audio editing, or serious gaming. If your box takes DDR, go for enough to push you to 512--it's still pretty cheap. Rambus is still a bit pricey, so if you're gonna trim yourself at 256, make sure it is quality memory with a high speed rating. Being able to run it at quad-speed should help overcome some of the slowdown a bit.
If it ain't broke don't try to fix it! But, hell if you can tweak that bastard just a little bit more....hehehe.