Why bother getting any 3.5e books now? (just wait a few months and there'll be tons of used books.....)
Basically seconded.
As for the books in question:
MIC & Spell Compendium: Useful to have everything in one place. Updates & revises some older material.
CompAdv: I use that one for its combat feats almost as much as the PHB and CompWar.
Races of...: I only own one (Destiny) and hate it. Too much space wasted on previously published races in ways that read more like campaign-specific info. I understand that Goliaths are pretty cool, though. Buy on discount only.
PHB II: like the not-mentioned CompChamp, it is most useful for its Alternative Class Features. Its useful, but not a "must have" IMHO. I got more out of Unearthed Arcana. Buy on discount only.
BoED: Its single most discussed bit of content, Vow of Poverty, is controversial to those who play RAW, and can be overly powerful if the DM doesn't read it. Neither it, nor its counterpart, BoVD, is essential, but they both have nifty stuff. Buy on discount only.
CompArc & CompMage: Both have useful feats. The older book has useful, though not indespensible, base classes. The newer has some good Alternative Class Features, including the very useful Focused Specialist ACF, and the nifty Reserve Feats. If all you're looking for is spells, though, get the Spell Compendium instead- it covers both.