If your shopping phase will be ended in the next days, AND this machine is not a professional production machine, I would use 7 RC till at least 29-09-09 when 7 is supposed to be released. It will save you some money but will require a complete reinstall of the system (at least if you want an absolutely trouble free machine) when 7 is really introduced. If you do not need Microsoft or Adobe support and can handle that yourself, 7 looks like the way to go, but realize that some drivers are not 100% stable, are not yet fully tuned or are still non-existent. That of course depends on your equipment.
OTOH if this is a production machine and troubleshooting is gonna cost you not only time but also money from clients, I would carefully consider the value of using a supported OS with CS4.
Vista 64 Home Premium with SP2 seems pretty well behaved.
I can't say that I have any complaints about it at this time.
Harm's point is a good one- if you want an OS that is well supported & has all the proper drivers available, you'll be looking at a bit of a wait for Windows 7.
I would recommend that you try out the Windows 7RC as it is free for one year! 7RC offers advanced security, enhanced performance, increased speed, great convenience and so much more. To learn more about 7RC and to receive direct access to the download center, head on over to Microsoft Springboard.
Microsoft TechNet / Springboard