Not much you could 'pimp' it here.
You didn't say which processor you have, but your socket is designed for old Celerons and Pentium III processors.
If you could find it, Intel Pentium III Coppermine (FC-PGA2) on 1400 MHz would be the best solution for that socket.
If you cannot find it, then Intel Celeron (FC-PGA2), same speed - 1400 MHz, could be good solution.
I recommend that you choose one module of 512MB RAM + your existing 128 module.
Or even better, 2 X 256MB modules of same manufacturer.
I presume your MB has AGP slot. You could find nice cheap NVidia FX 5xxx series card with 128MB, or even 256MB of VRAM.
Well, regarding HDD... I'm not sure that your budget is enough, but I would rather keep existing HDD and buy LAN card and Combo drive or DVD RW if you could find cheap one.
But, at the end, when you summarize all of this, you could still have problems with your power supply. I'm not sure that you could change it if you have brand machine. It's Dell right? Brand PCs suck!
Anyway, I hope this could help you a bit.