Because your current CPU is a 'single core' processor... The two you referred to are dual(2x3g) and quad(4x3g) core machines. Get with Win7 64bit OS so that you can use all the processing power and at least 4gb RAM...
I saw you kinda knocking anything past WinXP, but I think your opinion is stone-dead wrong on that. I am old-skool and die-hard (I still have DOS machines running for diagnostics), but I have *no* complaint with Win7 or 64bit... It will be a bit of a learning curve, but not terrible - and you have shown yourself to be above average in your understanding anyhoo... you will have no worries. Within a week, you will be wondering why you waited so long. :)
And i am not opposed to W/7, though i had Vista for a long time and am glad to be back w/ XP. Win 7 has new features and i appreciate improvements, but sometimes i find some old ways better, but you can get Classic Shell/ for some XP look and feel, and also XP mode if needed.