He is one of those players who has played a significant number of tests in both decades.
1990s was, statistically speaking, more bowler friendly than the 2000s where a whole host of batsmen have averaged more than 50.In the noughties, averaging more than 50 was expected of any good batsmen (Hayden, Gambhir, Hussey, Smith, Sehwag, Clarke, Sangakkara, Jaya, Mo Yo etc etc the list is quite long) . Where as, in the 90s, averaging over 50 was revered.
Having played 48 tests upto 31 Dec 2000, Kallis averaged a mere 43.09 runs in Test cricket. I'm not sure that can be overlooked while rating his career.
Interestingly, he had more responsibility as a bowler when he started, and he settled down to a batting all-rounder a little late in his career. So it is hard for me to call this. He has played in the same level as the best batsmen in the last decade though - If we take the period of 1st Jan 2000 to today. He has performed on par with Sachin, Ponting, Sangakkara etc and out performed Hussey, Jaya, Mo Yo etc. So this is a hard one to call.
For me...(I'm only listing present players and players from the very recent past)
Tier 1
Lara, Tendulkar (For proving that in their peak they were well ahead of the pack in Cricket, at a time when scoring was not as easy and also for staying with the lead pack in the flat track generation)
Tier 2
Ponting, Kallis, Dravid, S.Waugh, Hayden, Hussey, Sangakkara, Sehwag, Smith etc/etc.