i have a P4 630 and it handles all the games i play great!!! (HL2, Source, Q4)
the two will prob be about the same as far as performance in games, but the 650 will really shine when your working with other programs and doing light multi tasking! Hyper Threading is a HUGE plus. And be sure to go with A64 or EM64T.
AMD definitely. but i would spring for a dual core. the x2 3800 is only around $100 US more and the 4200 is around +$160.
wasnt there a patch for bf2 for dual core? or was it buggy? either way when it works, the dual core should be the better performer.