If you plan to overclock the CPU, than you'll want to get a premium heat sink and fan, like a Tuniq Tower or Thermalright 120 EX which run $50-60. Expensive, but well worth it in what you get back in OC.
For the Antec 900 itself, I'd recommend getting 2 more 120mm fans. It comes with 3 installed, but it doesn't include the side panel fan or the interior pass through fan, both of which are really helpful in cooling hot spots in your case.
For the chipset and VGA...you can probably hold off on those. The 8800GT fan isn't great, but its definitely sufficient if you simply increase the fan speed with good case cooling. For the motherboard chipsets, you can buy aftermarket stuff but if you get a board with a decent heat sink on both you might be ok. Otherwise you can always get something like the Antec Spot Fan if needed.
If you plan to keep everything for a long time, doesn't hurt to buy more cooling but if you're like me and constantly replace stuff, its not really worth spending $100-150 each time you build a PC for new cooling solutions. A good CPU cooler and case cooling is a must, the rest is optional imo.