May I suggest you try before you buy if possible.
Being in a similar position I opted for a more "modern" PC with windows 7.
Not a new one, but a good condition rebuild with dual core processor and solid state hard drive.
Definitely "quicker" and faster start up etc.
However and there always seems to be a however, I hated windows 7, every time switch on/of had to wait while it did automatic updates. Wouldn't run XP software, or outlook express etc. Nice Library but incompatible with any XP data in fact corrupted it.
Gave it a week but took it back and had Windows 7 removed and XP Professional installed, Much, much better.
Best of both worlds, decent performing PC and XP Professional.
Really think there is a scare campaign, to get everyone to dump XP, me, am going to hang in until the end.