don't count on being able to use you old hardware and simply upgrading your processor. trying to save money like that is usually not a good idea with hardware as old as yours. if you are a on tight budget consider saving up for an AMD system not INTEL. AMD generally has much cheaper hardware at the same performance and relaibility as a comparable intel pentium system.
that said you should consider replacing your motherboard, processor, and memory because it is most likely cannot be used with the new er motherboard. and possibly a new video card, sound card and possibly hard disk, a new case/ power supply, network card. well...actually thats everything, plus anything else i missed.
the bright side is that although this seems alot you should be aware that all this could cost you around only $320-400. i'll give you an estimate. if you buy these parts at
www.newegg.com and build it yourself:
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asus nforce motherboard with built in sound , network, video would cost $67 with free shipping
a new case with a 350watt power supply is $55 including $5 shipping
PC2100 256MB memory stick is $40-50
you could use your old cd-rom drive and floppy drive or get a new ones. floppy drive is $7 and cd-rom is $20-30
a AMD XP2400 processor which is equivalent to a pentium 4 2.4 cost half as much at only $82 retail with the included heatsink fan combo.
hard disk 40-80GB is $60-80
so everything is going to cost about $320 without a new floppy and cd-rom drive or $280 with them. this computer will be 7-10 times faster than that celeron
see, its not so bad is it?