I did a 512mb 6200 back in the day, it required a little special magic that I don't have the tool chain for anymore.
However the 6200 was a budget card, and the 9800 was a pro card. So it's really night and day as far as speed, with the 9800 winning on all counts.
I think the 9800 had a full 256bit vram path, and all 6200's were limited to 64bit paths. Not taking into account the clock speed of the Vram( faster on the 9800 ).
So the extra Vram won't really help in anything real world, because the data path and clock speeds are just too slow, but it looks really cool in ASP.
The GPU on the 9800 is also much faster.