Is it a good idea to replace CPU?

Hi there!

I am a student, and therefore VERY low on cash. I am looking around for an upgrade to My PC. I have an eMachines T4170, with P4 1.6 ghz, 256 mb RAM (I know, I need more ), and a GeForce4 MX440. I am planning on upgrading Ram/Graphics first, but I want to know if it is feasable to buy new processor for it? My motherboard is a socket 478 SIS L4S5MG, with a 400 mhz fsb. I am not that much of a newb that I dont know what kind of processor to buy tho! I was looking into P4 2.8 533 mhz fsb. Is it the right decision? I figured that it would probably be cheaper (and easier!) than buying a new motherboard/computer. However, someone told me not to do it because emachines Power Supplies are [-peep-], and it will fail. Is this rumor true? Will a new Power Supply help? Should I go with new motherboard? any ideas?

Or you could just buy a new compatible motherboard.... If you do that, make sure it fits in your case.

My Rig:
AthlonXP 2000+ 1.677ghz
GeForce Verto FX5200 128mb DDR
Maxtor 7200RPM 80GB, Stock 40gb
Kingston 384MB DDR
Cendyne 52x24x52 CD-RW
Lite-On 4x DVD-RW
Go EMachines...