OK, this isn't much of a post, just to put up in a public place my success in upgrading my old D845GRG desktop board. I had a S478 2.53 P4 and 1GB (2x512) DDR RAM. Opinions on the net and elsewhere said 1GB max and 2.8GHz tops. I came across some 1GB PC3200 sticks (out of a G5!) and a 3.06GHz CPU (SL6PG), both for little $$ (~$50 total.) Stuck 'em in and it all works fine.
Testing - Q3A timedemo, went from ~190fps to ~230fps (everything maxed @1024x rez)
and Handbrake seems a bit faster, maybe up from ~60fps to maybe ~75 or 80 now. Not bad.
Not worth alot of money, but for the little I spent it was worthwhile.