[RPG] How compatible are books for D&D 3.0 and D&D 3.5

An unofficial FAQ created by the community on Wizards' forums says the following:

The Psionics Handbook is from 3.0, and is no longer supported. The Expanded Psionics Handbook, despite the name, is a standalone system that replaces the 3.0 system. 3.5 psionics differs from 3.0 psionics far more than current edition magic differs from AD&D 2e magic. The consensus on these boards is that the changes were for the better.

This thread has some actual play feedback about the differences between the two. The consensus seems to be that 3.0 psionics were ridiculously weak (weaker than the base classes) with the exception of a few gamebreaking powers. Things seem to be better balanced in the 3.5 version and in general, much better received. Also, two new classes were added and some new psionic races.

From what I gather, you could use the 3.0 rules with 3.5, but given that the 3.5 rules are superior and released for free online, I'd say it's probably not worth doing.

(Note: If anyone wants to add specifics on crunch to my answer, or copy-paste my answer to use as the basis for a better answer, please do so. I don't know enough to be helpful!)

I presume your approach is this: You want to try to combine the 3.0 core rulebooks with the 3.5 SRD, presumably because you want to use the 3.0 books for source material and advice but the SRD for stat blocks and an authoritative system of rules. You want to know what will you miss, meaning what could trip you up because you missed the difference between 3.0 and 3.5. I'm going to explain from a GM's perspective, that is, that you'll be planning a campaign, and that you've familiarized yourself with 3.5 already.

In short, as long as you remember to always treat the SRD as the only source for crunch, you won't miss much. A very small amount of mechanical data is missing from the 3.5 SRD (such as the stat blocks for the few monsters that Wizards kept out of the SRD, and the PHB's XP table). You can use the old versions safely, when necessary, provided you're mindful of systemic differences (such as the change to Damage Reduction). It'll be easy to know when to be careful, though, because you'll be looking at a book instead of a website.

Let me dig in to each core rulebook for specifics.

Player's Handbook. Honestly, just tell the players not to trust the 3.0 PHB and you'll be fine. Everything that isn't crunch is common cultural knowledge at this point, such as knowing that fantasy dwarves are good miners and like to use axes. The XP table is all that's missing, I believe, and it didn't change.

Dungeon Master's Guide. I actually own the 3.0 DMG but not the 3.5 DMG, even though I haven't GMed 3.0 in years. In fact, I reference it all the time for my Pathfinder game, for its general tools and advice on how to GM. So I think you'll do very well without the 3.5 DMG and your players will never notice a difference, provided you always use the SRD for actual game mechanics similar material such as the magic item chapter. But, say, the table of 100 plot ideas will never be out of date, you know?

Monster Manual. Effectively you'll be ignoring it wholesale except for, e.g., reading the colorful monster descriptions aloud to the players, and that's safe since they didn't redesign any monsters in terms of concept (that I know of). There is the matter of the non-SRD monsters, but again, you'll know to be careful when using them, so you'll be fine.