Expreview published a preview of the ATI Radeon HD 3670 256MB (RV635). Performance-wise this card isn't really an upgrade over the Radeon HD 2600 XT - in most benchmarks there seems to be little difference but this could be related to driver issues.
From the roadmap of AMD, we can see HD 3600 family will take the place of HD 2600 series. The price will be about 100USD. User will ask how much is the improvement? Actually not much. The average improvement is about 3%. Let’s hope it is just a another Catalyst issue, and AMD will rise the actual performance of HD 3600 series by releasing a new version of Catalyst in the near future.
Check it out over here. The site also tested the power consumption and noticed that both in idle and load it's higher than the Radeon HD 2600 XT's power consumption.