You don't define precisely what you mean by these things.
* If we're talking about switches, forget it. Cisco does it better and faster, easier to manage, with more robust hardware and a better service plan (limited lifetime warranty on all fixed configuration switches!). A non-Cisco switch doing anything of value on your network is a surefire way to convince me that you are bush league.
* Routers - it really depends. What are you going to do? Just route traffic between LAN interfaces? A Cisco L3-capable switch will probably be the fastest for this job, considering that many of its traffic routing tasks can be done in hardware which has been made to spec. But if you're looking to stick with Linux, you can configure a Linux server with the hardware you require and load it up with a network protocol you need it to run. A Linux server can certainly run OSPF or BGP. However, what else are you need? Do you also need a firewall, a VPN concentrator, an intrusion detector, a WAN optimizer, a small phone system? Because if you need those things as well, a hardware router will do these things at once in addition to its routing tasks, with a better performance:price ratio. Configuring it is not hard to learn to do. If you don't have time, you can always phone someone else who's contractually obligated to fix it.
* Firewall - this is wide open. Every single piece of firewall software seems to approach things in a totally different way, especially in terms of management interfaces. I would look around for the one that communicates to you in the way you find most intuitive, and then buy the gear that runs that. While I know Linux on a server will have some powerful firewalling capabilities, I simply can't use most of the Linux-based management packages because they just don't seem to think the way I do. Hopefully this is remedied soon, because most firewall vendors are incredibly overpriced and, in the case of Cisco especially, occasionally hard to even obtain at all.
I'm no Cisco fanboy, although I do rely on them for my income (full disclosure). I also don't want to be a Negative Nancy, as I understand that not everyone warms up to the whole "you should be grateful to have our logo in your rack" attitude you get from Cisco...I certainly don't. But there is a reason beyond simple groupthink that causes people to buy their stuff - frankly, there just is no serious alternative when it comes to switches or multi-function routers.