I recently figured out that modern x86 cores at >2 GHz get about 6 times as much work done as an older SPARC CPU at 900 MHz (for a particular workload) - this is an older model CPU, but representative of what's installed at many sites.
This means you could replace a 144 CPU SPARC server, which must have cost $500,000 at least, with a 24 core x86 server giving the same throughput - one server from HP (quad Xeon, 6 core processors) is priced at $20,000. In the current economic situation, this is amazingly attractive for any new deployments - you may even find the old hardware's annual maintenance is more than the cost of the new hardware, so it's worth the hassle of switching an existing server.
Of course, your mileage may vary, and this ignores disk I/O, reliability features, etc - but you can get resilience through clustering these days, by buying two or three of these servers.
The only real options on x86 are Linux and Windows, in terms of what major corporations will specify and major ISVs will support (I know Solaris x86 exists, but how many people really use this compared to Linux?). Many companies who have used Unix for decades are far more comfortable with Linux than Windows for mission critical software, and their applications will port easily to Linux.
This is probably why I'm seeing a huge upsurge this year of enterprises that are specifying Linux for formerly Unix only applications. Far more of an uptick in switching to Linux than in the post-dotcom downturn.