now i cant give you the exact amount in pounds but i'll try and tell you video cards in 150$ range.a good one would be
from nvidia-
fx 5600
fx 5700
geforce 6600 agp(might be a bit higher than your budget)
geforce 6200(not the turbo charge version)
from ati
radeon 9600
radeon 9550
radeon 9700
x300/x600(if you can find an agp version)
well you can get any of these cards with atleast 128-256mb of video card memory.i dont think there is any need to buy more ddr ram(512mb) now,you might consider this when you have some spare cash.thats pretty much it,not exactly sure about uk prices but they should be close.the geofrce 6600 agp version, if you can find around your budget or even a bit higher would we well worth it.
to wotrop,..........which cards are you reffering to when you say nv 5500,i've never heard of them.there is a geforce 6200,6600,6800.are you by any chance refrerring to any of these or the gpu models.