Pentium 4 or AMD 64

Pentium 4 is generally faster for video/audio encoding. On the THG homepage look at the interactive CPU charts to compare processor benchmarks. AMD will beat Intel in games, hands down.

Personally I'd go for the 3500+ venice as its cheap, easily overclockable by about 10% with almost zero-effort (and much more with effort). It also runs cooler and is, in many opinions, a superior chip to Intel's.

Yet all that said, the Intel may be what you are looking for... only you can decide. Read some reviews, some benchmarks for the apps you will be running, and check those THG interactive CPU charts. The Intel will still be plenty good for gaming and the AMD will still be plenty good for multimedia tasks, keep that in mind...

P4c 3.2Ghz NWood / ABIT AI7 / 1GB XMS-Pro DDR 3200 / BFG GF-6800GT 256mb / Antec 380W

A64 3000 Venice / Epox 9npa-U / 1GB HyperX DDR 3200 / XFX GF-6600GT 128mb / Antec 330W

What do you mean by multimedia/workstation?
If you do not spend a high % of your time doing the type of video encoding that Intel is known for, the extra heat the prescott cores put out may cause problems. If by multimedia you mean watch a few movies, download some songs, maybe play around with a bunch of pictures, the A64 would be a better choice.

I didn't find your rig, but I found <A HREF="http://voodoopc.blogspot.com/2005/10/modern-day-tale-of-david-goliath.html" target="_new">this</A> which is very interesting reading.