Check microdirect.co.uk everyday as they have special offers each day on the homepage. If you check often enough there will be one for an Asus GeForce 4 Ti4400. It should be 99 ex VAT, because thats how much i got mine for.
any of the geforce 4 ti range is good, you could also consider ATI's new radeon range going from 9000 - 9800 pro, best cost/performance would be geforce ti 4200 or ATI radeon 9500 pro. If u want the best money can buy look at geforce 5800 fx or ATi radeon 9800 pro.