Cheap & PSU are too words that shouldnt go together. A 430W seasonic will save you about £5, but you really won't get a decent PSU for anything less.
If you drop the CPU down a grade, then you won't be able to get a dual-core. Is an a64 3800 really a worthwhile, if you have an a64 3000? I'd say stick with your a64 3000, buy the x1800, and upgrade to an X2 when you can afford it.
Another consideration in this purchase is the minimum spec of the x1800xt. It says it requires a 450W PSU. I know this recommendation is probably based on a high performance PC under full load with a bunch of extra HDs & peripherals - but its something you should be aware of.