Technical help required to choose socket A upgrade (Duron-Athlon-Sempron?)

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I have a bit of a slow machine and I want to upgrade the cpu.

At present I use a Duron 700.  I'm building up another brand new
PC but in the meantime I want to beef up this old one and give it
a cpu upgrade.  However I want to be careful that I work within
the limits of the existing PSU and therefore I don't want an
unecessarily power-hungry cpu.

Can someone kindly check out my thinking below.  

By way of background, I am currently running a Syntax mobo with a
Via SV266A chipset (the bus can be set to 100 MHz or 133 MHz).  
Memory is 768 MB of 133 MHz SD-RAM (256MB plus 512 MB).  The
current 700 Duron's specs say it is a Spitfire 1.60 volts with x7
multiplier on a 100/200 MHz bus with max thermal power of 31 Watts
and max current of 20 Amps.


I figure my options in descending order of preference are as

(1) Put in a low end Thoroughbred B chip: 1800+ or 2000+.
SPEC: 1533MHz or 1667MHz with 256K cache;  FSB is 266 MHz; 1.60V.
This cpu is predicated to run at a max of 60 Watts and 38 Amps.
Approx cost in UK is 35 for 1800+, and 40 for 2000+.

(2) Put in a Duron 1800 chip.
SPEC: 1800 MHz with 64 L2 cache;  FSB is 266 MHz;  1.50V.
This cpu is predicated to run at a max of 57 Watts and 38 Amps.
Approx cost in UK is 38.

(3)  Put in a Sempron chip:  2200+ or 2400+
SPEC: 1500MHz or 1667MHz with 256K cache;  FSB is 333 MHz; 1.60V.
PRESUMABLY I can run Sempron at FSB of only 200 MHz and
     therefore get only 80% of its power.  IS THIS CORRECT ???
This cpu is predicated to run at a max of 60 Watts and 39 Amps
     but will the max be 80% of that because of the slower FSB?
Approx cost in UK is 36 for 2200+, and 46 for 2400+.

Are these options in the right order?  Are there any other options
I am overlooking?  Are there any important good points and bad
points with these options?

In fact, it is hard for me to get a TBred-B chip except from eBay
(which I do not want to use).  So how much of a compromise is
taking option (2) or option (3) above?

Thanks for any info.

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