319 seems like a weird amount of ram, but I bet that this is probably 320MB after conversion from units of 1000 to units of 1024 ( Ie million-bytes compared to mega bytes or 1024^2 bytes ). With a 256MB and a 64MB ram stick you will have exactly 320MB of system memory.
RAM is pretty cheap to buy, and in my opinion a PC as powerful as a 2.50Ghz Pentium 4 should have come with no less than 512 MB of ram as a minimum.
Anyways, Moogle has some excellent advice, shutdown all the applications that have icons in the windows systray, you will see in the windows Task Manager resource tab ( on WinNT4, Win2000 and WinXP ) how much free ram you have, and this should increase with each program you shut down.
Even things like complicated wallpapers, virus scanners, open webbrowsers or disk browsing windows take up memory that planeshift could be using.