Buy now.
If you are buying a tower, my guess is you are not buying it to run iTunes and iPhoto.
None of the Apple's own Pro apps are universal at this point.
Same can be said about most third party programs.
Some of those (especially specialized) will get recompiled only in a couple of years from now. A case in point: I own a license for Mac Spartan 04 (a high end quantum chemistry app). That thing is PPC only, and is not getting Universal until, maybe, 2008.
Also, Intel has not even released a new desktop CPU yet ("Merom" is coming out in the 3Q 2006, last time I checked).
So get the G5 tower - you won't regret it. Their performance is quite competitive. And you will enjoy this great performance over your whole upgrade cycle. Software is not going Intel-exclusive any time soon...