Clawhammer, Winchester, and Venice are different core revisions. Clawhammer is the oldest using a 130nm process, then came Winchester on the 90nm process, and the latest is Venice with a 90nm SOI process.
Now first of all, you should avoid the Clawhammer. From what I can see, the only Clawhammer 3000+ is only available on Socket 754. It is actually a 2GHz part so its will be faster in some cases than Winchester and Venice but it is uses an older motherboard so you can't upgrade later. As well, it lacks dual channel RAM support. The 130nm part that worked on the current Socket 939 is called Newcastle. Regardless, the 130nm parts use more power and generate more heat.
The coolest part is actually Winchester. However, you should just get the Venice. It's the newest and while it runs slightly hotter than Winchester its still cooler than the Intel equivalents. As well, Venice has some optimizations that should theoretically improve its performance over Winchester. In reality the improvements aren't significant but they're still nice to have.
Now about HT. It allows 2 threads or processes to run at once using unused processor resources. This is beneficial in encoding tasks which have almost all been HT optimized by now allowing them to issue 2 threads at once. The benefit to multitasking is that threads from 2 applications can be processed at once. However, HT isn't as good as a real dual core setup as HT can only process the 2nd thread if there is spare processor capacity while a dual core will always be able to process 2 threads.
If your price range is for the A64 3000+ then you should stay with that model. Intel doesn't have a comparable part in that price range as the 2.8GHz 52x are very slow.
If you are looking for encoding performance then look at this chart.
http://www.tomshardware.com/2005/11/21/the_mother_of_all_cpu_charts_2005/page32.html
A64 3000+ Winchester 7:19 min
A64 3000+ Venice 7:17 min
A64 3200+ Winchester 6:40 min
A64 3200+ Venice 6:38 min
P4 520 Prescott 6:26 min
P4 530 Prescott 6:04 min
P4 630 Prescott2M 5:59 min
This is for converting VOB (DVD rip) to Xvid.
However, for gaming AMD's are faster.
http://www.tomshardware.com/2005/11/21/the_mother_of_all_cpu_charts_2005/page24.html
In Wolfenstein even the A64 3000+ Winchester beats the 630.
If you mainly encode, Intel is the better option with the 630 priced similar to the 3200+. If you game, AMD is better.
Of course Pat is correct about power consumption and heat. If that is also a concern of yours, then AMD is better.
Generally overclocking is safe as modern processors all have safe guards to prevent permanent damage. However, if you are not comfortable with it and aren't planning on getting better cooling then don't bother. Just concern yourself with a better graphics card. For your tasks I recommend an ATI X1xxx series one. They are available in a variety of price ranges. While their gaming performance is good, what is most beneficial for you is their hardware accelerated video encoding support. ATI's new 5.13 driver should be available on December 22 to activate those features.