MOTM

I have at least two of each filter.  I rarely use the 420s in 
series.  Normally I run them in parallel with slightly different 
settings for a stereo effect.  
I would recommend getting a 440 next, then a 490 and finally a 410 
before getting a second 420.  
Each filter has its own character.  I think the 440 is the sweetest 
sounding and I use it the most.  If I want a more agressive, harsher 
sound I use the 490, like for a lead that has to cut through a dense 
mix.
All Paul's filters sound great.  I'm anxiously awaiting the 480, 450 
and 470 as well.