It really depends on what the PC will be mostly used for. If you are into games or image editing, the faster the machine the better. Indidently, if you intend to use Windows XP, I would recommend you up the RAM to 512MB of DDR, as XP tends to like memory.
If it were me, I would go for the XP1500 as you should see a difference. Alternatively if you can do a "hands on try before you buy" then you will see the difference before you part with your hard earned.