Replacing the hard drive will do nothing for gaming performance. What it will do is make the system possibly seem a bit more responsive when loading programs and Windows itself, along with give you extra capacity.
Your board uses the nForce 1 chipset, which is quite good actually. If you update the bios, you could put up to an XP 2600+ in there. Your board also supports a dual channel DDR memory interface, which you can only take advantage of if you have 2 sticks of memory installed. Assuming you only have a single stick of memory (a 256 MB stick of PC2100 DDR memory) and assuming you want to get the best bang for your buck, this is what I would recommend:
Switch the CPU to an Athlon XP 2000 or 2100+. This alone will increase system speed dramatically, and more importantly allow you to take advantage of a faster video card.
Replace the graphics card with a GeForce 4 Titanium 4200. The 4200 series cards cost half the price of a 4600 card and deliver 85% of their performance. A graphics card upgrade is the other absolute MUST for top performance for you.
If you only have 1 256MB stick of memory, add a second 256MB stick. If you already have 2 128 MB sticks, do not fill the 3rd slot as it will actually reduce memory timings to ensure stability.
If you hard drive is running out of space, switch to the 40 GB drive. If you have plenty of room left, consider the hard drive of secondary importance.
A 2000+ CPU in the retail box will probably cost around $100 at a local computer store. (You could get it cheaper online, but I tend to prefer supporting a local store.)
A GeForce 4 Ti 4200 card should be around $140 - $199 depending on the amount of memory onboard and other features (TV out, video in, etc)
256 MB of PC2100 will cost about $50. Only do this if only 1 stick of memory is currently in the system. Going to 512 MB from 256 MB will increase performance slightly.
A 40 GB drive would probably be around $90 for a good brand (Seagate or Western Digital). As I say, this is of secondary importance.
It is worth mentioning that the chipset drivers for you motherboard are much more mature for Windows 2000 and XP than 98. This is IMO, not a bad idea.