Have you tried using Windows Media Player to stream your Sage recordings to your Xbox? Not at all as pretty an interface, and I'm not sure what format Sage records in, but if it works then it's saved you the hassle and cost of an upgrade.
To answer your question, there is absolutely no functionality difference between XP and Vista Media Centres. Only difference is that Vista has a much nicer interface and better plug in support, which enables things like Media Browser, but there is no out of the box functionality difference.
However, if you don't have the power to run Win7 then I'd also like to suggest that you don't have the power to run Vista. Win7 is noticably lighter on system resources than Vista, though Win7 Media Centre does place heavy demands on your graphics card.
Finally, if you're determined to upgrade then don't waste time with Vista, you'll end up regretting it. Take the plunge and go for Win7 as it actually does offer some new functionality.
Win7 Ultimate RTM x86 | AMD64X2 3600+ | 3Gb Ram | 1.8Tb HDD | ATI4650 | Hauppauge PVR150 - Virgin Media cable | Pinnacle 3010ix - Freeview DVB-T | Pig ugly big black case | Xbox 360 wired extender