Can I put a Celeron 2.5 in a Dell Dim 4500S?

I have a Dell Dimension 4500S with a Pentium 4 1.8. I have a Celeron 2.5 that seems to run much faster than the P4 in another system. Can I move the Celeron into the 4500S? (The other system maxes out at 1 GB RAM and has a 5400 RPM HD, whereas the 4500S maxes out at 2GB RAM and has a 7200 RPM HD.)

The 2.5Ghz Celeron ran on pocket PGA478, the same as the early Pentium 4's, so it should work properly. Run CPU-Z, go to the Mainboard tab, and see what sockets the motherboards run. If they're the same, then you can safely swap them.

Dell: P4 Northwood Socket 478 mPGA (this is all from the CPU tab)
Compaq: Celeron Northwood Socket 478 mPGA

I noticed towards the bottom there is a "multiplier". The Celeron multiplier is 25 whreeas the P4 is only 18. So, does this imply that the Celeron is indeed faster and it'n not something else?

The Dell can take DDR 400, 333, or 266. (Max 2GB)
The Compaq can only take DDR 266. (Max 1GB)

Will all DDR memory clock back to what ever the MB can handle? This stuff is expensive and I'm thinking I saw DDR 400 1gb cheaper than DDR 266.

The documentation denoted above says that the system can accept single or double sidded RAM. Is this related to "Low" and "High" density RAM? If I could buy HIGH I could get it cheaper I think.

The Dell is a small form factor tower.
The Compaq is a full Mini tower.

I cannot find the PSU rating of the Compaq unless I pull it. I have the Dell 4500S, and a Dell Dim T500 PSU as welll.

So, is there anyplace that I can feed in the different options to see which set of parts gives the best performance, or do I just keep switching and testing and ignoring my family - or find smart folks like you?