The performance gap is noticable, but I wouldn't there is a geat margin of improvement. However the GF2 MX does have a much better feature set, and also things that could help long term like texture compression and better multi-texturing speeds, and also has T&L which maybe one day will be useful.
EDIT: Referring back to the original question. HDD or graphics card? Depends which is in most need of upgrade. At the moment standard HDD sizes are tending towards 15-20 GB in size. I would say that if you are nowhere near that area of size then upgrade the HDD.