This is pretty much how my upgrade started.
I decided I wanted a better processor, so as an AMD guy I was naturally looking at the socket 939 CPU's. That meant getting a new motherboard and since PCI-Express is now teh shit, I went for a PCI-E mobo and thus had to get a new gfx card. Then I thought I will probably need a better PSU, so off to look at them. And since I was getting all this, I thought why not get a new case too.
Since I had three 512MB ram sticks, I decided to leave one of them in the old system. So then to get the old system running too, all I needed was some hdd for it. Bought one used and so I now have two systems:
Old system:
AMD AthlonXP 2600+
MSI K7N2-Delta
Club3D Radeon 9600pro
Maxtor 60GB ATA
Apacer 512MB PC3200
LG 52x CD±R/RW
Eurocase MDL5412A BLACK w/ 350W PSU
New system:
AMD Athlon64 3200+
DFI Lanparty UT-nF4 Ultra-D
Club3D Geforce 7800GT (replaced the original cooler with a Zalman VF700-Cu)
2x Seagate Ultra-ATA 200GB
2x Transcend 512MB PC3200
NEC ND-3500A 16x DVD
±R/RW
Seasonic S12-430
Compucase Midi-Server 6919
Have to mention about the new CPU that it is supposed to be a temporary solution. I wanted to get an Opteron but since AMD wasn't shipping them anywhere when I bought the new stuff, I decided to go for just some decent CPU at the moment and then get an Opteron later when they're available. Now they seem to be available but I don't have the money just yet