Do you play games? If so you will see the difference a better graphics card brings. Your motherboard supports 4x AGP maximum, which is what your current card is so don't go for an 8x specifically, but more graphics memory will be a huge benefit. I recently added just a 64Mb GeForce MX440 to a PIII 600Mhz that had a 32Mb TNT2, and the difference was quite great. The processor did keep the performance back a little though with recent games.
Why do you wish to go for an external hard disk? You could fit an internal one easily enough, saving you money for more RAM. An 80Gb ext is about £130, whereas an OEM internal about £70. You would need to upgrade to USB2 to use the external one properly too, costing even more (£25-30).
The components you are choosing *should* be compatible (as long as you get the USB2 card for an external disk).
If you want to go a little further with the upgrades, an Athlon 2000 would be about £45, motherboard £40-80, and new case (with bigger power supply needed and fits most motherboards) £40. Internal 80Gb 7200rpm ATA133 HDD about £70, Gainward graphics £55, 512Mb PC2100 £55. In all £305-345 for a much more up-to-date system.