If your forum database is going to be in the same machine, then you can do well with a Pentium 4 2.66 provided you get a fast disk, it has to be a SCSI. Or perhaps you can get the server with two disks, and you can get one disk devoted to the database. Also, you will need 2GB RAM. Both disk performance and RAM are more important than sheer CPU speed for forums, but the Celeron is crippled at the cache level so it's no good (also avoid Sempron on the AMD side, although it's better than the Celeron).
In terms of performance, as you can see in this chart by Tom's Hardware, the Pentium 4 2.66 Northwood core (which is most likely what you are getting) has exactly the performance of the Pentium 4 2.8 Prescott core (the most common at this point), which transitively means that the 2.66 is better than your Celeron 2.8.