Caddy for your existing drive.
http://www.scan.co.uk/products/newlink-35-cdlsb-805-aluminium-pata-hdd-to-usb-20-enclousure
Replacement internal hard disk.
http://www.scan.co.uk/products/320gb-western-digital-wd3200avjb-av-ide-%28pata%29-7200-rpm-8mb-cache-89-ms
This is the cheapest and simplest option; you can get a 320Gb IDE HDD for ~50 quid. Question is, coming from an 80Gb disk, do you suddenly need 1Tb of storage?
Your alternative would be to migrate to SATA. A SATA controller card that will fit your board (which I’m guessing is PCI only – if you’re PCI-E equipped then it’s cheaper) comes in at around 35 quid. You’ll also need a power adapter for the disk. You’re then looking at around 40 quid for a decent drive in a 1Tb flavour.
PCI SATA controller:
http://www.scan.co.uk/products/lycom-st-125-raid5-sata-ii-4ports-%281external-esata-plus-3-internal%29-inc-dual-pci-brackets
Power converter:
http://www.scan.co.uk/products/20cm-serial-ata-power-converter-rb-414-4pin-molex-to-15pin-sata-%28normal-psu-to-sata-sataii%29
1Tb HGST drive:
http://www.scan.co.uk/products/1tb-hitachi-deskstar-0f10383-7k2000c-sata-3gb-s-7200rpm-32mb-cache-89-ms-ncq-oem
One advantage of moving to SATA is that you’re future-proofing; if you move to a SATA-only motherboard at some point then your IDE disks are going to be useless (though I suppose you could move it to that caddy). It does work out at double what you were initially going to spend, though.
Two caveats:
1) You really only want to ever be connecting one SATA drive in this manner. The Molex > SATA adapters are a frig at best, and you’re going to start flooding the PCI bus if you go beyond a single drive.
2) These products are off the top of my head based on assumptions in order to give you a starting point, rather than the results of extensive research. There may be more suitable / cost-effective options.