Hey Jules,
Your zt3000 is still a very nice notebook, and well worth upgrading. As you've suggested, the hard drive is currently the weak link and since your 40GB Fujitsu is just a 4200RPM drive most anything will be an improvement. Unless you're very sensitive to battery life I'd go for a Seagate or Hitachi 7200RPM drive, though, as that'll yield noticably faster boots & program loads than the 5400RPM Toshiba.
Looks like you're shopping for parts at Newegg and they're a great choice. I've ordered from them maybe 2 dozen times and have never had a problem. Fast shipping & great prices.
This Hitachi 7K100 is what I just upgraded a friend's Inspiron 6000 with and we're very pleased with it. I've had the 60GB version in my Thinkpad for ~18 months & it's been excellent.
http://www.newegg.com/Product/Product.asp?...N82E16822146052
512MB of RAM should be plenty for WinXP unless you run applications that are memory hogs like Photoshop or you commonly have half a dozen or more aps open simultaneously. If so, 1GB is worth it, else you probably won't notice any performance improvement from it. I don't think your zt3000 supports dual-channel memory, so there's no issue with being sure to have 2 matched DIMMs installed, either.
Should you need to add memory, though, I believe you need PC2100 DDR CAS-2.5 SODIMMs (PC2700 proabably works, too).
In lieu of added memory you might consider adding either an internal DVD+R/W drive, or getting an external USB2 DVD+/-R/W drive for backups and movies. The chepest good option is usually an NEC ND-6750A if it will fit the zt3000 -not sure. Else you can probably find the factory DVD+R/W drive on eBay.
For external, NEC ND-3550A is great & USB 2.0 case (BYTECC seems to work well).
To mirror your current drive to the new one and for backups I like Acronis True Image -works great. $30 from Newegg.
http://www.newegg.com/Product/Product.asp?...82E1681279691SF
Of course, a new HD is a good time to consider a clean install to clean house. If you've been using a WinXP box a couple of years it's usually collected enough garbage and Registry clutter to benefit from a clean start.
Don't forget to keep a current Firewall/AV package updated.
-Tim