The FX line is comprised of the 5200 through 6850 (the 6800 series is starting to trickle into the market).
The 5200 being the slowest and the 6850Ultra being the fastest.
With very few exceptions, the 256MB versions of video cards use slower ram and perform slower than the comparable 128MB video card version. The current exceptions to this are the ATI 9800XT and the NVidia 5850Ultra (more new ones are on the way in this category).
Video cards use very expensive ram, so to keep the purchase price within reasonable limits, the video card companies compromise the performance by using cheaper, slower rams, on the 256MB video cards.
In general, you get what you pay for, in terms of video cards today. If you pay $80 U.S., or less, for a video card today, you really need to lower your expectations of what that video card will perform like. Ignore the marketing on the box as it is there to entice you to purchase it.
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DISCLAIMER: This information is provided 'as is' and does not imply a recommendation for any given product.