Depending on your perceived urgency for a new computer, you may consider waiting closer to Windows 7's release. First and foremost, Windows 7 is, as Steve Ballmer described, 'Vista only better' with increased performance, additional functionality, and greater ease of use while maintaining both the same hardware requirements and existing software and hardware compatibility. If you purchase a system now you will undoubtedly desire an upgrade to Windows 7 in less than a year's time, although at that point you would have to pay an extra $100-$300 for it. Therefore, unless you have an immediate need or desire to replace your existing system I would recommend waiting on your anticipated purchase.
I would also like to note that there are two unconfirmed yet realistic rumors circulating. The first is that Windows 7 will ship to the general public before the end of the year, making its debut before the holiday shoping season, which Microsoft sorely missed upon Windows Vista's release. The second is that those who purchase computers running Windows Vista July 2009 onward will receive a free upgrade to the corresponding Windows 7 edition upon its release. Therefore, you may only have to wait six months before making your acquisition rather than the nearly-full year till Windows 7's release.
Regardless of the accuracy of those rumors, however, the computer manufacturers and stores will offer greater discounts as Windows 7's release date approaches as they attempt to liquidate their stock of Vista-based systems. Those potential deals alone may make the wait worthy, and the free upgrade to Windows 7 only sweetens the pot.
The ultimate decision is yours, but I wouldn't make a snap decision at this point.
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