This question is a bit old and possibly stale at this point, but I thought I d toss in an answer for future views on the subject. After all, as much as I like CE 5.0, its a bit old and stale itself. :)
First off, understand that Visual Studio 2005 and the CE 6 dev environment are two distinct entities. While CE 6 runs as something of a plug-in for and depends upon VS 2005, the converse is not true. So if you’re doing CE 5.0 application development, you can just as easily do it with (standalone) VS 2005 as VS 2008, so long as you have a suitable SDK with which to link your Smart Device project.
As for developing for or porting to a CE 5.0 platform, you’ll definitely need to stick with the CE 5.0 bits only. Consider CE 6.0 to be a completely different OS with a completely different memory model, user/access paradigm, etc. Microsoft provides the Standard SDK for Windows CE 5.0 as a free download, available here:
http://www.microsoft.com/downloads/details.aspx?displaylang=en&FamilyID=fa1a3d66-3f61-4ddc-9510-ae450e2318c3
Note that the SDK linked to by JoelHess is for Windows Mobile 5.0, which differs in significant ways from Windows CE 5.0.
Alternately, you mention an SDK from the device manufacturer…this type of custom SDK typically includes all or significant portions of the Standard SDK plus the device-specific components required to get your platform up-and-running.
A final point: you only mention developing an application targeted to CE 5.0, but if you also need to be able to build the platform image, you’ll need Platform Builder 5.0. A 120-day eval edition is available from Microsoft (unable to post a second link due to my obvious newb-ness).
