The lower amount of memory mainly has an effect in higher resolutions (1600x1200 and up). Since a 512MB equivalent doesn’t exist, it’s hard to get specific numbers.
I found a few articles that may shed some light on your options:
This ExtremeTech article compares the 3850 and the 256 MB GTS. The 3850 is sometimes 50% faster, sometimes 100%.
At TechPowerUp, the 8600GTS 512MB, 8600GTS 256 MB, and 8600GT are compared. The difference between 256 MB and 512MB is mostly negligible. The GT (with DDR3) is slower.
At Madshrimps, the 8600GT with the fast memory (DDR3) and slow memory (DDR2) are compared. The DDR3 card is between 0 and 10% faster in benchmarks.
HardOCP shows the performance of Hellgate: London using a 3850 at 1600x1200. It’s very playable.
From what those links, and your provided links, tell me, I’d end up with two contenders that offer the best value for money: the cheapest 8600GT ($109, DDR2) and the 3850 (should be available for $179). At 1280x1024, they’ll both deliver adequate performance on every current game but Crysis.
To confuse you further, there is a third option: an 8800GT with 256MB of memory should arrive on the scene any day (two weeks ago, NVidia said it would arrive “in two weeks”), with a suggested price point between $179 and $199.
And to finish, you should also consider the power draw of the cards. The 8600GT and GTS draw 30-40W less than the 3850 and the as-yet-hypothetical 8800GT 256MB, according to the ExtremeTech article I linked above. There is a slight chance that the extra power draw will overtax your computer’s power supply.
And of course, I could be wrong.