Thank you for your question.
Yes, Photoshop 5.5 IS upgradeable to CS2. This page at Adobe lists the specifics:
Adobe Photoshop CS2 Upgrade
To install this upgrade successfully, you will need a licensed version
of Adobe Photoshop CS or Adobe Photoshop 7.0 or earlier, on the same
platform as this purchase.
NOTE:This upgrade does not apply to Adobe Photoshop Elements,
Photoshop Album, Photoshop Limited Edition, or PhotoDeluxe licensed
Download Windows Download File Size: 350.4 MB, read before buying
Macintosh Download File Size: 451 MB, read before buying
Either costs US$149.00
CD-ROM Version: Windows / Macintosh US$149
It appears from the above that a licensed version of Photoshop is all
that is required to be installed on the target computer and any
version will comply. And, as you can see, it can even be downloaded
from Adobe's site if you have a high speed connection.
DPReview also notes:
"...Licensed users of any previous version of Photoshop can upgrade for $149..."
The upgrade lists for $149, but can be had at a discount if you shop
around. For example. Pricescan shows small savings available down to
CNET shows it available for as low as $128
I trust my research has provided you with the answer you were seeking
. If a link above should fail to work or anything require further
explanation or research, please do post a Request for Clarification
prior to rating the answer and closing the question and I will be
pleased to assist further.