You didn't mention that in your original post.
I'd double check the specifications for each card - the 256MB card is typically a fair bit more expensive, so I suspect either a variety in specifications or a mistype on the price.
Barton 2500+, 512MB Corsair Platinum XMS 3200 CL2, Radeon 9700, WD Raptor 10,000 rpm S-ATA HDD, Asus A7V600, Enermax 460W SilentPlus PSU.