dual-core will be VERY good for video encoding; gaming will either ignore it or use it too.
Right now I wouldn't advise you to go for an Intel chip (your mobo is Intel only) if you're to use dual core: P4s (and Pentium Ds) are severely limited in that area. If your friend can wait, get a Conroe instead - that is, if you're hell-bent on using Intel.
I'd recommend an Asus A8N-E with an Athlon X2. Slightly less performance in video encoding (however it's nothing to sneeze at either, as it can encode 2 MPEG-2 streams concurrently in real time if asked nicely), and kicks asses in gaming.
This is without overclocking, if you do O/C you can get a 20% boost pretty easily on air cooling.
My friend want to buy a new rig, with tight budget, he ask me to make some spec for his rig, what do you think about this one:
ASUS Radeon X800 PCIe 256MB DDR3
Corsair Value Select 2x256MB DDR2-533
Seagate SATAII 120 GB
ASUS DRW 1608P
other hardware he'll use from his old rig.
He'll use this new rig for gaming and video encoding, and i'm still confuse which CPU will have better performance for that task?
Well, a 3200+ Venice is a good match for the 630 performance/price wise but the 805D is a niche by itself (only low-end dual core out there) since AMD's entry level dual core (X2 3800+) is an higher-end CPU and priced accordingly.
The X2 3800+ is quite good at overclocking: due to its small L2 cache, it has a much higher chance of succeeding in reaching high frequencies. I pushed mine from 2000 to 2400 MHz hardly breaking a sweat (just had to revert my RAM and CMOS settings to factory defaults), and I compiled a Linux kernel on that: no problem.
I would try and push it further if my BIOS wasn't restricted to 240 MHz max FSB...