New
#21
No worries mate.
You should have no problems in replacing the HDD. That motherboard has two SATA connectors. *NB SATA II is backwards compatible with SATA I which I believe your board is. (This means that new HDD's will still work with your PC )
For the video card, it will need to be a AGP x4/x8 card and NOT a PCI-E card. (for example) :
Newegg.com - Computer Hardware,Video Cards & Video Devices,Desktop Graphics / Video Cards,AGP 4X/8X
As for the memory, apparently that board can technically take a maximum of 2GB (2x1GB) , but some models are limited to a maximum of 1GB. Unfortunately it appears as if your board falls into that category:
Maximum allowed 1.0 GB (2 X 512 MB)