the biggest Sonar Bundle i've ever made is about 450mb. During a project, I often hit ctrl-alt-del and see how much RAM Sonar is using. It rarely goes above ~250mb.

It's got absolutely nothing to do with your bundle size.

It's got everything to do with your personal workflow & methodology

I frequently run into RAM limitations on my XP32 set up because I tend to use a lot of RAM hungry sample streaming synths, like BFD2, EWQLSO, Sampletank etc.

If all you ever do is record audio then no, you won't need a 64 bit system.

But if you're doing anything more complex with Midi, then 63 bit is the only way to go

Note: your bundle may only be 450Mb, but, and this is assuming you're using the "Per Project" option in Sonar (which you should be), you can easily check the actual size of a project by opening Windows Explorer, navigate to a project and right click it.

I'd be surprised, well, flabbergasted, if you had ANY which were less than a Gig.

I would start by saying hello being that I am new to this forum... Hello!
With that said... Do not minimize the importance of memory. Even though your previous projects were small and not hardware intensive,  there is something to be said about having the "headroom" to do more.  This "headroom" plays an important role in the stability of the OS/Application/Hardware interaction. It also helps by allowing the machine to work "easier" which translates into cooler.  This potentially has an impact on the life of the machine. 

The other consideration is obsolescence and the constant march (part innovation, part marketing I admit) in the evolution of technology.  With this march comes the possibility to do more, work more efficiently and potentially faster on platforms made available at the consumer level. It is necesaary at times to take a cautious but open outlook in order to realize these advantages. Also all of these products are built with a limited life cycle unfortunately.  In order to take real advantage of the new offerings, some updates will be necessary.  This is done while reducing/eliminating support for the older platform (again in this case... mostly marketing).
 Lastly, Soon, all new offerings will be 64 bit with rewrites of the more popular older choices.  Now is a good to to lay the groundwork for these offerings (Hi mem/HD space - 64 bit OS and Apps) while keeping your older projects available for recall if needed...

Onward... I Say!!!!
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Sonar X! Producer- Win 7 64- I7 2600k - Asus P8P67 - 16gb 1300 DDR3-SB Xfi HD sound card (optical out to mixer)-  Cosair 120gb SSD (OS -Audio APPs)- 2TB Seagate (Storage/Content)- Casio WK 1600 midi controller- Akai MPD 26 midi/DAW Controller- Edirol M-16X Digital Mixer/DAW Controller  

Yeah, i went with two xeon processors so i have 8 real cores altogether.

i spent the rest of the day researching DPC latency (latency mon is GREAT) and got it tweaked so that it barely shows on the graphs. good lord theres a lot of info out there, and nvidia seems to be the culprit (thanks to that other thread!)
anyway, i cant wait to see how this sits with projects. i hope this one lasts me another 5 years or more.

Some comments:

"It's got absolutely nothing to do with your bundle size. "
>>>um, wrong. but it does! the bundle size is which files windows needs to address your project at any one time.
i mistyped it though, its not really the bundle size when youre working with cwp projects. to be more accurate, i should have said ctrl-alt-del will tell you exactly.
...and 100% of my projects (and i have thousands on my HD's) are less than 1mb in size. most are around 300kb.

"and this is assuming you're using the "Per Project" option in Sonar (which you should be)"
>>>nope im not. i put all of my audio into a single directory, always have and probably always will--- it 'just works' and so i know i can always find audio if i crash or lose it somehow.. if i need to bundle and export to send to someone, bun or OMF files work for me. 
mostly, i hate the idea of naming a song before i even make it (seems ridiculous to me)

Always update drivers from the manufacture's website and the intel driver update helped me tighten up some friend's machines. BIOS, firmware for your ssds, and tweaks and a lean install will keep a box at minimal latency.

CPU-Z, Crystal Disk... there are many tools to help eval a system.

My Win7 machine with 12GB of ram is just as stable as any XP machine I have had with tons of headroom for effects and virtual instruments. 64bit seems a lot better for me in both 8.5.3 and X1