Hello James,
I would definitely get 1GB of RAM; if not more. As Nick mentions, WinXP (and Win2K/NT before it) need a lot of RAM, as do many programs and files.
I
think that you are confusing Centrino with Celeron? A Celeron is a P4 that has only a small amount of cache memory on it, and therefore will lack in relative performance to the Pentium's. The Centrino is the name of the mobile system (which include the Pentium M CPU and the matching motherboard chipset), and is an excellent laptop system.
I would recommend the AMD Athlon 64 CPU's, over anything from Intel, for a desktop system, though. They are much cooler running, and are if anything, a little more powerful for many functions. They also tend to cost less, though certain large OEM's who have special deals with Intel can certainly sell you high value systems for low prices.
Another concern is to get a quiet system -- the byproduct of powerful computers is heat, and to cool them well, the case must be very efficient and have good air flow, so as to avoid the need for high RPM fans. Some systems are quite loud and become very annoying to work with; and need to be avoided.
Another point that you should consider, is getting a pair of hard drives in a RAID 1 (or mirror) array. This functions to make a full copy of the main hard drive, and so if it fails, then you can still boot the computer, use it, and not lose your data. You can then replace the failed HD, and the mirrored pair can be rebuilt. It's the redundancy that can save you a lot of trouble. You should also fully back up your data -- I would do this onto another computer and/or onto an external HD, that you can then remove from your office for safe keeping elsewhere.
Lastly, I would highly recommend getting a video card that has dual outputs, so that you can use dual monitors; either now or in the future. Dual monitors is a wonderful thing -- it greatly expands the ways that you can use your computer. I refer you to
this thread, and also
this thread on system specs, too.