Fading Suns is a great setting. I haven't had the chance to play it using D20 rules so I can't comment on their conversion. The setting is good enough to make it a worthwhile purchase however.
Other settings you might consider are Heavy Gear and Jovian Chronicles. The latest edition for both games includes D20 modern conversions. HG is a militaristic setting set thousands of years in the future on a planet called Terra Nova. Technology is advanced but other than some war vehicles (more on that later) it's not very different from a present day setting. Heavy in political flavor it involves two factions on Terra Nova that continuously skirmish. They do come together when Earth decides to invade. No aliens. Great storyline.
Jovian Chronicles is different in that it takes place in the 23rd Century. The solar system has been colonized (for the most part) but no faster than light travel exists. Various factions exist based on some of the planets with Earth and Jupiter being the focus of most of the setting. The setting is much more "space based" than Heavy Gear. The storyline is much lighter in this setting but enough is there to allow any GM to come up with adventures.
Both HG and JC make use of mecha. In HG the "gears", as they are called, are only four meters tall on average. Most of the combat takes place on land with the Gears functioning as very agile tanks. They do not however replace tanks. In fact, in a tank vs. gear battle most tanks will wipe the floor with the gears but there are balancing factors.
Jovian Chronicles on the other hand, takes place mostly in space. Mecha are called "exo's" and also allow for agility on the battle field. Most exo's are very large 15 meters or so.
I should mention that it is very, VERY easy to completely do away with mecha completely in either game if you don't like that sort of thing.
You can read more about both settings on the website:
www.dp9.com