If you’re still curious with the runs, 4.3 was the last 16-bit version, and Office 95 was the first 32-bit version. The Office Suit in 64-bit is a big step forward with support to 64-bit OS which apparently have a bit thin support so far.
Thinking big on this market side of 64-bit adoption, not only MS Office is widely used, another thing other vendors are likely to follow in Microsoft’s footsteps when it comes to offering the next version of their products. Office 2010 and Vista’s step to 64-bit computing may slides up to expected future 64-bit integrations and developments.
Before Office 2010 arrives, we still have something to look forward as Microsoft sets April 28th ship date for Service Pack 2 for Office 2007.