Wait for d20 Future is also an option
D20 Future (August 2004) will have psionics and magic in a futuristic setting based on d20 Modern rules. Note that you must wait it released, and that you also need to buy d20 Modern to use d20 Future. Last note, IMO d20 Modern classes are overpowered.
Dragonstar is DnD in far future with wizards, sorcerers, drows, dragons, etc. in a high tech world with starships. I have the two books (Starfarer's Handbook and Galaxy Guide): well, I don't want to play it. For me sorcerers, elves, dragons, remind me too much of Tolkien and medieval heroic fantasy. When you throw in laser guns and starships, it becomes confusing, a bastard sort of things. Noneheless, on Fantasy Flight Games website is some pdf previews to see by yourself.
I don't know about Farscape d20, and don't know if it is still available in stores.
Star-Wars d20 is to play Star-Wars, nothing more, nothing less. It's a bad idea to buy it if you plan to use it as rules for an entirely different setting.
Finally, there is... MY OWN SYSTEM !!! hehehehe.... But mmmh... sorry, but it will be available in a long time from now. However, since I have the same intent as you, I have been compiling and doing a d20 rpg based on d20 Modern + Traveller 20, plus tids and bits of d20 Star-Wars, Dragonstar, Bllod & Space, and d20 CoC. Well, if you can wait for a full year, it will be available for free: 30 classes (15 bases, 15 prestige), 200 feats (from many OGC sources), magic as in CoC, but also with prestige class of mage who doesn't lose sanity when casting spells (works as a DnD wizard or arcane advanced-class of d20 modern), plus many things. But well, you will have to wait unfortunately (despite classes, skills, and feats available in june or july).
Well, what can I say more? Only that Blood & Space is mediocre, but for its price you just add it to basic DnD rules, and create your homebrew campaign with it.
Turanil