It sounds like you are not just weighing the pro’s and con’s of blade servers vs. traditional server but you are also facing a decision on physical vs. virtual servers. Let’s start with the blade vs. traditional server topic.
The main advantage of blade servers is their density: you can fit many more blade servers in a 42U rack then you can traditional servers. Another advantage is the improved cable management that blade servers provide, you do not need to run cables to each individual server with blades only to the main chassis. Traditional servers on the other hand have advantages: more capacity for local storage, more expansion slots, less complicated management and multiple USB ports and a physical optical drive.
The choice between blade and traditional servers often comes down to personal preference and by what type of server is already in use in your data center. Blade servers have evolved in recent years and now offer comparable hardware options as traditional servers. Older blade servers where typically limited to only 1 or 2 single core CPUs, 2 NICs and no support for Fibre Channel storage, some modern blade servers can support up to 16 NICs, 4 quad core processors and multiple Fibre Channel or iSCSI HBA adapters.
Also, you need to consider the cost of replacing all your (perfectly functional) servers. Most people who go for server “compression” solutions (blades and virtualization) are adding lots of server- next time you have to add an application server, evaluate whether you can justify the larger price tag of a blade chassis by evaluating your space, power, and cooling envelope.
Having said all that I would say if you currently have traditional servers and do not have space issues in your data center then I would stick with them.
Now as far as the physical vs. virtual question. Virtualization offers a lot of advantages over using physical servers, you do not need a SAN to use it but many of the advanced features require it. Regardless of your physical hardware choice I would definitely recommend looking at implementing virtualization in your environment, especially if many of your current servers are under-utilized. Be forewarned though that there is a learning curve for implementing virtualization and if you have no experience with it you might consider some training to learn more about it before you dive into it.
One of the common driving factors towards virtualization is hardware support costs. As servers age and their support expires, virtualization can be a method to ensure that all of your servers are running on supported hardware without paying for support on too many servers. Generally speaking, with ESX we see consolidation ratios of 4 VMs per CPU core. With an average new server of 2 quad core CPUs, we see about 30 VMs on a single physical host (although it is important to scale the memory to accommodate this many machines).