Precision T7400, Which verison of Windows 7?

Hello,

I writing about a Precision T7400 for a customer of mine.  It has two Intel Quad Core Xeon X5482 Processors at 3.20 GHz, and I think it has 8 GB of RAM, but maybe only 4 GB.  The Intel Products web site says that this CPU has a 64-bit Instruction Set, and under Advanced Technologies, it says Intel® 64, so I figure that makes it a 64-bit Processor.

I'm writing to learn which version of Windows 7 to use; 64-bit or 32-bit.  Just checking.

It has an nVidia Quatro FX3700 512MB Video Card, and has an SAS-6 160GB SATA Hard Drive, but this computer has never performed well.  The computer has Windows XP Pro (32-bit) with SP3 on it now (we opted to downgrade from Vista Business), but for all of this computing power, it's horribly slow for a computer of this caliber.  His Secretary has a Precision T5400 that was bought at the same time, with a single Xeon 2.0 GHz Quad Core CPU, that runs circles around his.

I recommended this Precision T7400 to him because my Ex bought one on my recommendation about 1 month earlier, with only a single Xeon Quad Core 3.20 GHZ CPU and the SAS-6 Hard Drive, and it really screamed when it was new (for about the first 6 months), so I thought that with two Xeon CPUs and the 10,000 RPM Hard Drive, it would really perform, but like I said, it's horribly slow (and so is my Ex's, by the way).  His computer takes 12 minutes to boot and fully load everything from Power Off.  From Power Off, it takes the SAS-6 boot-time configuration utility 3 whole minutes by itself to load before Windows ever starts to boot.  12 minutes is a long time for a computer of this caliber. 

I worked on it a month ago and got it down to 6 minutes at Power On from 20, but a few days ago I tried to make it faster again, and after I got the latest BIOS, some new Drivers, some Windows Updates (including IE8) that he had ignored, and ran some Optimization programs and Registry Cleaners, it's up to 12 minutes now.  Granted, there are a lot of programs installed, as it's a business computer, but this computer should be able to handle Anything!!!  So I'm thinking about taking out the SAS-6 Hard Drive and putting in a regular SATA 7200 RPM Hard Drive to see if that will help the performance, and while I'm at it, I want to upgrade to Windows 7.  I'm quite sure that a fresh installation of Windows will help, and Windows 7 is pretty efficient.  And I'm fairly sure that replacing that SAS-6 Hard Drive will help.  Any thoughts on this?  I'd appreciate your input.

So back to the original question...  32-bit or 64-bit Windows 7?  He has many new programs, like Office 2010, and even though they're all 32-bit, I think he'll be OK with his Software Library if Windows 7 64-bit is the way to go, but we'll have to check with some of his business partners to see if the programs they supply are Windows 7 64-bit compatible.  And we'll have to check with QuickBooks.  Maybe Windows XP Mode will help with those programs.  I'm thinking about Windows 7 Ultimate.  This computer was purchased in May of 2008, so I don't think there'll be any issues with Windows 7 compatibility, and I have Driver Genius Pro, in case Dell doesn't have Windows 7 Drivers.  However, if we decide to stick with Windows 7 32-bit, for compatibility with some of his business-partner-supplied programs, do you think there'll be much of a performance loss as compared to 64-bit?  And if we go with Windows 7 64-bit, does Windows XP Mode work with that, and does it create any performance losses? 

Just trying to cover all the bases, and finally get some performance from this machine.  I'm hoping you'll address all of the questions.

Thanks in advance,

Craig