i'd recomend more that 256mb of ram. the comp will go much faster with 512mb
This is quite true, as now-a-days, 512mb is becomming the minimum requisit for RAM to thoroughly be able to run programs.
Celerons are the lowest end model of the Intel Series. The P3's are the higher end, but are the last model they made, this is why they are at P4's now...Athlon-XP is something entirely different made by a different company although their XP series is the same as P4 basically in the sense that the XP series is AMD's top performance line (except of course for the AMD64's that are now out, which THOSE are absolute top of the line) but anywho, the XP's are basically the previous top-line processors, where as Durons, are AMD's low end. AMD Duron is like the Intel Celeron, both are low end. Next up you have P3's and XP's probably would rank together the best, then P4's and AMD64's...
However, don't let Intels higher clock speeds fool you. A 3GHz Intel isn't necessarily faster than a 2.2GHz AMD because AMD's do more instructions per clock cycle than Intel.