In my experience, the size/performance, of the heatsink/fan has not increased between these two CPU's.
Mainly because the original AMD 64 300 CPU's were based on a larger and hotter core design (die size has now shrunk and as a consequence the CPU's run cooler) And.....Athlon 64's run pretty cool anyway so the retail coolers that is used operates well within it's cooling capacity (in other words it has headroom for a faster CPU)
As long as your case is well ventilated (extra fans) just keep an initial eye on temps...make sure the X2 4200 stays below 50C (which I'm sure it will)
Martin
On wings like angels whispers sweet
my heart it feels a broken beat
Touched soul and hurt lay wounded deep
Brown eyes are lost afar and sleep