Dunc, i always thought that getting a card with more RAM was better, and i'm hell bent on getting a 256 card now, but apparently you might find that 128 is still ok (? sounds weird, but people are claiming that some 128 cards are better than any other card in the price range)
E.g. for around £100 apparently the best card you can get is the nVidia (i know..) 6800LE. It's 128 Mb, but it works very and if you have some knowledge (i.e. google) you can mod it to run much faster.
6800 cards.
The X700 card you mentioned sounds nice though. I love Saphire cards (my 9800 is a Saphire. It overclocked to a stable 450 MHz on aftermarket cooling, an increase of 50 MHz 👍 ). So.
Sounds like each card can be overclocked, take your pick. I can't guarentee either is good of course, but i'm sure both are better than your 9800.
Personally i'd take the X700 as the appeal on 256 Mb is still strong in me. And i love Saphire cards.
More X700 cards.