Re: Upgrade
It will definately be cheaper to buy a faster Pentium 3 processor than buying a Pentium 4, because upgrading to Pentium 4 requires too many decisions to be made and will ultimately depend on how much you got to spend.
But I don't know what motherborad/chipset you use. If you are using a socket 370 (FC-PGA) motherboard you probably can use a Pentium 1GHz, max.
I doubt your existing motherboard supports the Pentium 3 with the new core, that uses a lower voltage than the older ones, in other words i don't think you can use the Pentium 3 1.1Ghz or higher.
You'll have to check you motherboard manual to check if you can even use the Pentium 3 Ghz.
If you buy a pentium 4 - you definatley need a new motherboard.
Maybe new memory, unless you buy a motherboard that requires DDR, you old memory will be useless.
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Warning - I can be long winded - but i hope it helps.

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*Operating Systems* - I advise not to use WinMe - its bug heaven. Even with Service Packs - its still crap. If you intend to use Win2K Pro (for whatever reason) then u should dual boot it with Win98SE. But If you decide to use WinXP then there isnt any point dual booting with Win98SE. WinXP will run all software that require Win95/98 using the compatibility mode. If you do move to WinXP - its best to make sure that all your other hardware devices have XP support drivers, likewise with Win2K.
I'm UK based so unless you are too - I can't offer any websites that would give good deals on hardware. But If you are unsure of things and need to read up on certain things, w*w.tomshardware.com and w*w.sharkyextreme.com are good sources of information with hardware reviews.
*Processors* - Unless you're like Woody - Die Hard Pentium Fan. I would advise checking out the AMD Athlon XP. It is definately worth more your buck than the P4. And Yes the Athlon does out perform the Pentium is certain areas, the same is true reversed though.
P4 2200 Vs Athlon XP 2000
h**p://www6.tomshardware.com/cpu/02q1/020107/index.html
If you intend to overclock, you will need to buy a better cpu fan, otherwise the manufacturer supplied fan is more than adequate.
Athlon Vs P4 o/c'ed - bloody insane P4 @ 3000
h**p://www6.tomshardware.com/cpu/02q1/020128/index.html
As for *motherboards*. I'm not sure how many other manufacturers have motherborads that support UDMA133. I know MSI certainly have UDMA133 support on both Athlon XP and Pentium 4 boards. But there will be more sooner or later. (BTW UDMA100/133 is also referred to as ATA100/133)
If you decide to keep your existing memory modules (assuming they are SDRAM) Intel has i845 motherboards that support the slower SDRAM memory. I *THINK* there may be some motherborads that support 2 different types of memory (well there used to be with older chipsets) - so it might be worth checking them out (if there are any) just for future upgradability sake.
Check the link for P4 motherborad reviews h**p://www4.tomshardware.com/mainboard/02q1/020130/index.html
Even though this is a review on VIA vs Nvidia Chipset - It does look at some Athlon XP boards.
h**p://www4.tomshardware.com/mainboard/01q4/011126/index.html
*Harddrives* - Personally i prefer IBM, but western digital have drives with 100Gb & 120Gb and *8 Mb Buffer*.

A word of warning though if you buy a new harddrive, unless the older 15Gb drive uses the same UDMA mode as the new drive i wouldnt put them both on the same IDE channel/cable. If you do the harddrive using the slower UDMA mode will slow down the data transfer for the supposedly faster harddrive.
It might also be wise to buy harddrive coolers.