I've asked about this on a few forums and what people have said is that for today's games you won't see a big difference but for titles released in the next year it might come in handy. Some people too have told me that by the time 256mb is required, the video card will need to be updated anyway. Personally, I've decided to wait until the reviews come in and see what they say about the advantages of 256 MB.
I think if I do go 256MB, I'm going to buy the 256MB Radeon 9800 Non Pro version - I saw a picture of the Sapphire box at Gamers Depot - since I'm cpu limited to 2.6 ghz which would bottleneck me. Also, I just can't afford $750 Canadian for a video card so hopefully the non pro will be alot cheaper.