I'm happy with my Athlon 64, but I can't offer you much by way of comparison, since I don't have a P4 system. I will say that cooling hasn't been a problem with this system, or with my Athlon 2600 system--I used one of Thermalright's big copper heat sinks on that system, and had extremely stable temps that stayed around 35-38 degrees C. I'm using a Thermaltake heat sink on my Athlon 64 system--a Venus 12 with a custom fan. The fan that comes with it was so loud that it sounded like a helicopter was trapped in my case, but I liked the heat sink (it has more fins that Thermaltake's other Athlon 64 heat sink), so I just took off the noisy fan and replaced it with a silent fan.
I think the Athlon 64 3200 is a really good deal right now at ~$230; the 3400s are about $310, which isn't a lot more, but I'm not sure you'd see enough difference in performance over the 3200 to warrant the extra cost. My A64 3200 is noticeably speedier than my Athlon XP 2600. Cinema 4D renders measurably faster, especially in scenes that use CPU-intensive rendering tricks such as displacement mapping. Now if Maxon would just make C4D v9 a 64-bit application (I think chances are good that they will). Most games ran fine on my 2600, but I did notice an improvement in Thief 3's performance on the A64 system.