Its the same old story really, if you dont want to do any tinkering, just plug & play then fork out the extra for a mac.
If you dont mind getting your fingers dirty then go for a similarly speced windows machine.
If you want the bare minimum and save yourself a packet then get a second hand dedicated windows laptop. You can get by on the pentium4 dual cores and a couple of gigs of RAM.
Dude I'm using my Mrs' laptop which is like 5 years old and really slow but serato works like a charm, never had any problems with it at all, little bit gittery at points but its all good. I don't think you need a mssively specd up comp to run it to be honest. Obviously if you can get a nice one go for it, my mates macbook air runs so smooth.
You do notice performance increases in serato when you use a decent machine. For example more RAM means tunes get loaded into serato quicker rather than waiting a few seconds for it to load. And more processing power means the machine doesnt struggle to process the timecode signal into a place in the tune being played.
At the end of the day a higher specc'd machine will always be better, but doesnt necessarily mean its needed.
If you do want to go for a macbook air or similar, i really do recommend looking at other kinds of ultrabooks like the lenovo ideapad or acer aspire s3 etc. Cant stress enough how much of a waste of money it is to buy a macbook if all you reallllyyyy want it for is to run a DVS.