Is your P4 a Willamette (Socket 423) or Northwood (Socket 478)? (I don't think Intel made 1.8 GHz Willamettes, but PC133 Sdram i845 boards for Socket 478 aren't that common.) The short answer is that the 2.4 Celeron would be better than a 1.8 Willamette overall, but probably no better than a 1.8 'A' type Northwood.
The Celeron processors at 1.7 GHz to 2 GHz are Willamette derived Socket 423 CPUs that offer absolutely miserable performance.
The 2.1 GHz and up Celerons are Socket 478 models derived from Northwood P4s (basically they run on a 400 MHz FSB and only have 128 kB of L2 cache). This means that best case they can offer similar performance to a 'A' type Northwood P4 (the 400 MHz FSB models), but because of the extreme reduction in cache size they usually don't perform very well compared to their P4 cousins. The reduced L2 cache size can really hurt performance in many games and other apps.
The biggest bottleneck I can see with your system is that it's using PC133 Sdram.
