Win7 64bit... and all the RAM you can afford.
You will be well satisfied in the next 30 days or so... (the learning curve is operationally mild, but finding and using settings and new features might take a bit).
Win7 lasted less than 2 days before I deleted it and put XP back on. Its 2 major flaws were bad running of one of my major programs and unable to print to a network printer, even though it downloaded the driver, installed it and recognized it correctly (it would always say that the printer was offline). I probably could have worked around the first problem with an XP VM but the 2nd killed it for me. Easier to delete the partition and start over while I didn’t have a lot of stuff reloaded.