Set up my dual boot Windows 98 & Windows XP system on my VIA Nano U3400 800MHz / VIA VX855 Chipset Thin Client with 32GB CF card. Windows XP (which was installed 2nd) was a breeze, installed without issue and all drivers installed and setup fine. Did my usual test of Tomb Raider: The Last Revelation, output at 1080p with maximum settings. A bit laggy, but playable. If I dial down the settings to something more reasonable this will be a fine retro gaming system.
The Windows 98 install was less successful. It took nearly 3 hours to install, then it had issues with drivers & also laggy mouse movement etc. Plus the impossible hurdle - this thin client uses onboard VIA/Realtek HD Audio and not AC'97. I'll probably just keep this thin client on Windows XP.
Also started working on my 2nd Thin Client, with Via C7 1GHz / VIA CN700 Chipset.
Managed to squeeze in a 30GB mSATA SSD with mSATA-SATA adapter, on the left hand side:

The SSD is covering the IDE ports, but as this is such a small enclosure there is no room to fit anything to them anyway. Oh and the bottom and sides of the adapter have been insulated.
Going to also set up a dual boot 98SE / XP system on this. However it has a major issue at present - the CPU is overheating. I've had to temporarily attach a USB case fan, otherwise it simply gets unstable and unusable after about 15 minutes. I'll sort this issue properly this week, will clean the heatsink and add some new thermal paste. Failing that, the motherboard is regular Mini ITX and has a standard CPU fan connector onboard, so I can replace the heatsink with a small heatsink/fan - which will require some modifications of course.