Whats the difference between a standard AMD XP and an AMD XP Thoroughbred?

Is there any sites that list the details?

Should I go for a Thoroughbred or the standard AMD XP? Will the Thoroughbred work on a Abit KD7 KT400 M/B?

The original Athlon XP's were based on the Palomino core. These were fabricated on a .18 micron process, and had a default voltage of 1.75v. They scaled up to the 2100+ at which the architecture proved to be limiting. The Thoroughbreds are based on a .13 micron process, allowing them to reach far higher speeds with lower amounts of voltage, and as a consequence, generally dissipate less heat and draw less power. Far better for overclocking. The base of the T-Bred's die is also significantly smaller than the Palomino. They scale up to the 2800+, with defaults of 1.45v, 1.5v, 1.6v and 1.65v.

OK I used to have a Tbred 2400+ Which I thought caused my computer to crash, I swaped it to Duron 900Mhz which seems fine, I couldn't RMA the 2400+ because they found no fault with it. Could it be incompatible with my motherboards? I tried it on 2 different KT400 chipsets. In my manual it only has AMD Athlon / Athlon XP / Duron. Does that mean it doesn't support a Tbred??

The KT 400 chipset supports tbreds I see no reason why it wouldnt work.It could be possible that your power supply is too weak for the 2400+

Agreed. All KT333/400 mobos support T-Breds AFAIK. The problem lies with either your power supply or cooling. Normally, when a motherboard does not support a certain architecture, you're not going to be able to get it to boot in the first place.

http://members.cox.net/alpha_03/installation.htm

If you list your power supply like funnyperson1 pointed out, you may be on to the source of your problem...

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2100+ AIUHB 0248 MPMW
256MB Crucial PC2100 + 2x 256MB Kingston (Hynix chip) PC2100 @ 6 3 3 2
[152] FSB x 15 = 2280 MHz @ 1.8 Vcore with memory frequency at 100% [152] @ 2.77 Vdimm
37(min) C to 46(max) C Winter ; 42(min) C to 50(max) C Summer
Epox 8RDA+
Thermalright SK-7 with variable speed 80x25mm YS-Tech FD1281259B-2F
BFG GeForce4 Ti4200 8X 128MB; Antec SX-835II case ; 350W Antec SmartPower SL350

I have a cheap 350W PSU.

that could well be the reason for your problems, also it could be the ram, get memtest86 and run it (even with the duron) and see if it fails after a few tens of minutes if it does, then its your RAM..
But as you said the duron works fine, then the PSU might well be the culprit