Hi there,
I have built quite a large lab myslef now, so Im happy to offer some advice:
Im currently studying for a CCNP/SP, and then VP.
First and foremost it all depends on how far you want to take your lab. Mine has become a hobby/addiction.
In terms of routers, 2600's are pretty cheap now @ about $100 AUS a pop. They are a great start as they are modular, so you can mix and match. If you cant afford these, look at getting 2503's. They offer 2 X serial ports, and an ethernet port. They have all the needed capabilities for CCNA. Both these will take you into CCNP no worries, but for future proofing, you should go 2600's, or 3620's/40's. Try to stay away from 2504's. They have token on them, and is no longer used. I only have then in my lab to provide me with serial bridges.
You will need a frame relay switch (router really). You can do this two ways, either buy a 2600 and slab an 8 port serial card in it, or buy an older 4000 series switch and use that as a frame relay switch.
In tersm of switches, not a great deal has changed since IOS 12.0 on the 2900's. Sure there have been changes, but for the basic cisco cert stuff, its all mostly the same. If you are on a budget, source some 2924's. They do vlans, trunking, ISL, etc. All the basics. If you can stretch it, get some 2950's, or 3550's. The price jumps a lot here though, and that may be something you consider down the track. Stay away from 1900's. They are useless, and beware of ebay sellers saying they can be used for CCNA. They are junk now. Old, and no longer used.
Thats enough to really get you started. In terms of a PIX, its really not needed as in essence, its just a bunch of access lists compiled on the one device. If you have a firm grasp on ACL's, this is something you can purchase later on. ASA's are the go now, but PIX will still be in the industry for a long time yet so its good to know.
And finally, ISDN. The ugly duckling. Everyone avoids it, but its again great to know. The ONLY way you can play with ISDN is if you get an ISDN simulator device that acts as the TELCO. You can then make calls back and forth between your routers. Although ISDN has been removed from the CCIE lab as far as I know, Its a vital cisco skill, and a must. ISDN sims are not cheap, and most people dont add this to there lab. I just bough one yesterday for $500 AUS. Its the first one thats become available on ebay in Australia for the last 18 months. :>
I have in my lab the following:
3Com Surestack 24Port Hub (for ethereal, packet sniffing, etc)
Cisco 2924C-XL Switch
Cisco 2610 Router (with 10 serial ports! Frame, etc)
Cisco 2503 Router
Cisco 2503 Router
Cisco 2504 Router
Cisco 2503 Router
Cisco 2504 Router
Cisco 2504 Router
Cisco 1900 Switch (used for spacing only. Never actually turned on)
Cisco 2924C-XL Switch
Cisco 4700 Router (More Frame, etc)
Cisco 4500 Router
Cisco 4500 Router
Cisco 4000 Router
Cisco 2924C-XL Switch
Cisco 2924C-XL Switch
Cisco 520 PIX (version 6.3)
Cisco 1900 Switch (used for spacing only. Never actually turned on)
Cisco 7940 Cisco Phone
Cisco Call Manager 4.2 Publisher
Cisco Call Manager 4.2 Subscriber
ISDN Simulator. 2 BRI, 2 PRI
Hope this has helped you, and someone else!
