The only thing that makes old computers worth collecting is two things: 1 - it has some kind of unique hardware that isn't available any more & 2 - It runs software and games that are difficult to run these days, holding nostalgia value.
So the way I see it, anything Pentium 4 & newer = junk, just slower iterations of what we have today, nothing unique.
A quick list of worth-while valuable old PC technologies:
CPUS:
Pentium 3 Tualatin (As the core was famous for being extremely efficient, it was re-implemented into the intel 'core' series after the P4 failed)
Pentium 2 (The cool slot 1 form factor holds a lot of nostalgia for people, unique)
The first AMD Athlon Slot A (The cool slot A form factor holds a lot of nostalgia for people, unique)
Pentium Pro (The original pentium but with onboard cache, they were extremely expensive and used their own socket 8 motherboards, unique)
Motherboards:
The 440BX chipset and most notably the Asus 440BX (famous for its absolute stability, compatibility and overclockability)
Asus TUSL2 (The definitive Tualatin motherboard, which had modded performance bios's released)
Graphics cards:
Rendition Verite (One of the first 2D&3D inclusive designs, unique in the way the core was re-programmable like an fpga, also had the very first 3d accelerated port of quake Vquake)
Nvidia NV1 (Nvidias first graphics card, used quad rendering rather than triangles, just like the sega saturn, very unique)
Nvidia Geforce 256 (The worlds first GPU implementing hardware based transform and lighting, previously was offloaded to the CPU)
PowerVR PCX1/2 (The first tile based rendering graphics card, very efficient - also cool since powervr is used in modern smart phones)
PowerVR Neon 256 (Unique as it was the same or similar technology as used in the Sega Dreamcast)
Matrox G400 MAXX (The most powerful direct3d card that competed with the voodoo3 and nvidia tnt2 ultra, also introduced environmental bump mapping)
Matrox Parhelia (Famous for its use of fragment antialiasing and extremely high quality 10bit colour)
3dfx Voodoo 1 (Famous for being the first fully featured 3D accelerator for OpenGL, Direct3D and Glide, supporting all period features and blending modes, 3D only card)
3dfx Voodoo 2 SLI (Famous for being double the performance of the previous card, plus introducing SLI scanline interleave, linking two cards together for double speed)
3dfx Voodoo Banshee (The first successful introduction of the 3D voodoo core and 2D into one chip, also unique as the GPU and RAM mhz could be changed seperately)
3dfx Voodoo 5500 (The first dual chip graphics card solution, also introduced hardware T-Buffer effects and RGSSAA anti aliasing)
Sound cards:
Aureal A3D Vortex (The first hardware HRT sound tracing card with absolutely amazing 3D sound, was bought by Creative labs and squashed)
Creative labs AWE64 Gold (The ultimate ISA dos card for quality, compatibility and features)
Creative labs X-Fi - original models (The last of the old>modern sound cards that still supported hardware based features and the windows HAL hardware abstraction layer in Windows XP, after XP 7 onwards hardware 3D sound was removed)
This is just a rundown from my own memory so don't shoot me down if I made the odd mistake.