I supposed your memory is DDR. Since you're mentioning to buy the faster one so it's mean you'll buy DDR2, right ?
If yes, then you'll need to say goodbye to your 478 processor.
For a slow upgrading, you can check the Mainboard such Asrock Dual VSTA or Asrock 775 ( i don't really get the names, just check Asrock).
One or maybe the two of them support AGP, PCI-E, DDR and DDR2 in one mainboard.
Buy the processor such E6300 or P4D. You still can use your DDR and VGA card for amoment, when your budget is raising, buy the DDR2 then you can say goodbye to the DDR.
The board that broke was an Asus P4s800, and my graphic card was a fx 5200 agp version, after these two components broke i took out the cpu, hdd, and disk drives. I just left them around as spares, but this week my bro wanted me to build him a gaming rig which is able to play most games on medium, so i went out a bought a Geforce 7600 GT.
I think i'll buying a new box anyway so i not so worried about the power supply.
At the moment these are the components i already have:
Before you go purchase the E6300 & say the Gigabyte GA-965P-S3 which fits your budget, what PSU do you have? But with a $300 budget, you are not going to get a C2D, a new mobo and a gig of ram. Currently, memory has shot up around %50 percent. The same memory I bought last year for under 80 bucks is around 120 nowadays. You're going to have to add another 130 for memory if this is the case.
This is very important. If the PSU isn't powerful enough than your purchases will be done in vain.
You don't need DDR2 to run games. Upgrading your DDR400 from 512MB to 1GB will be fine. I have an Athlon 2400+ with a gig of RAM & a 6600GT. It allows me to play Neverwinter Nights 2 but on lowest setting. It's playable but I need a better video card & CPU. I'm hoping to get a 2500+ and OC it to 3200+.
You're best bet might be to get an Athlon 64 3200+, a relatively cheap mobo (ASUS A8N-E) and an extra 512MB of DDR400 Ram. This PQI Turbo DDR400 with a MIR will keep the entire update to under $200.
Unfortunately, I can't upgrade to even such a basic Athlon 64 even though I have a 350W PSU. I only have a 20 pin connector and the motherboards for an Athlon 64 require a 24 pin connector. Make sure your PSU has a 24 pin connector but based on the age of your computer, I don't think this is the case.