welcome to the forum!
1: the D20 is rated 1db quieter than the S20. depending on where you have your system located and the internal components used, your results will vary. for example, five 15K RPM SAS drives and a loud video card in a D20 will be louder than one 7200 RPM SATA drive and a quiet video card in an S20. the answer here is, "it depends." 
2: the S20/D20 are compatible with windows 7. drivers for each system are available on lenovo.com. if you purchase a system with a DOS license (which is how my D10 was configured), your hard drive(s) will arrive blank and you will get a driver CD for windows. the latest drivers are online so the CD won't be necessary.
3: third-party memory will work just fine. lenovo offers their own memory for customers who need their memory to be supported under the system warranty. when choosing memory, it's best to match speeds. also, uDIMMs and RDIMMs cannot be mixed.
4: regarding ubuntu, you may be interested in the linux discussion here on the forum. i haven't seen many thinkstation users running linux simply because it lacks support for typical applications used in the industry. lenovo does offer some linux drivers for the thinkstation lineup but i can't say to what extent an S20/D20 will be supported under ubuntu. admittedly, i haven't touched linux since 15 years ago when redhat and slackware were unknown names and it took half a day of hacking to get Xfree86/X11R6 working correctly.
Re: a few questions on thinkstati
i haven't heard the FX 580 personally. from experience, i can say that the smaller the fan, the louder it is. for example, my FX 4800 is silent compared to the FX 370 that came with my D10. of course, nvidia could have improved acoustics with the latest refresh.
windows 7 will more than likely install most if not all of the drivers you need -- some of which will come directly from lenovo. when i installed server 2008 R2 on two of my systems, i only needed to update a handful of drivers. it was actually quite automated.
with 6 GB i would recommend going with three 2 GB modules if possible. the difference between 1+1+2+2 likely won't be noticeable compared to 2+2+2 -- it's just better to match module sizes whenever possible.
if you plan to use SATA RAID, you'll be using the onboard intel ICH10R controller. this is known as "fake RAID" since it uses the CPU to control data processing. considering that your CPU is markedly faster than any dedicated RAID controller processor, onboard RAID tends to be quite fast. plus, if your system planar (motherboard) ever dies, your array can be moved to a new planar without having to rebuild. about the only benefit to using a dedicated RAID card is battery backup. but, if you keep your system on a UPS (which is recommended anyway), the risk of writing bad data to a software RAID array is very, very minimal.
on a personal note... if your budget allows, you may want to consider running two SSDs (SLC if possible) in RAID 0 for your OS and two SATA drives in RAID 1 for data. your system will be extremely fast and still have some redundancy. you'd need to put your user folder on the SATA array since it risks being lost on the SSD array in the event of a drive failure. or, since SSDs are quite fast as-is, you could run the SSDs in RAID 1 (which will give RAID 0 read speeds and single drive write speeds) and still benefit from the redundancy. food for thought.
