hp pavillion xl753 (old, i know, but it was free!)

my question is: does anyone here think it would be worthwhile and cost effective to upgrade the cpu and ram? my system is already customized, with 320 MB ram, 2 hard drives (30 GB each), and 2 CDRW drives, but i want to add a better sound card and a USB2.0 PCI card. .. the motherboard and CPU are stock, and most of the extra hardware came out of another hp pavillion, that was about the same age .. the CPU is a 668 MHz Intel Celeron processor in a socket 370 .. .. would it be better to scrounge up some extra $ to get a new machine, or can this one be upgraded to run better?

the reason i asked this ? is my cpu tends to max out every time i listen to any audio and do anything else .. and the internet is my ONLY source for news, radio, video (even though i don't watch a lot) , and i have almost all my music on the computer .. .. when it freezes, my stereo is worthless .. .. i also listen to 'BlogTalkRadio' a lot, and usually go check out other pages in additional tabs etc ..
maybe i should be asking if this question has already been answered in another thread, and if so, can anyone point me in that direction?