The BIOS you use does indeed have some somewhat-major effects on how your card overclocks.
Depending largely on the RAM that your card has, the stock BIOS (the one it came with) might not be the best one for your card.
For example, my card has Samsung 8E-GC2A RAM chips on it, at stock voltage with my card's stock BIOS, my stable RAM limit was ~384, with the Giga-Cube BIOS it went up to 409, and a volt mod added another 5 MHz on top of that.
9800 (NP/Pro) video cards have had a lot of 'revisions' over time; different RAM chips have been used, different PCBs have been used by diffferent manufacturers, and presently even different cores and parts intended for more expensive 9800 XT cards are being used in 9800 Pros, to save costs by shutting down fabrication of old, out-dated hardware, like R350 cores.
As such, the BIOS your card ships with is
almost certainly not the best one for your card. A few examples, if you have an R360 with an XT PXB, you want to use an XT BIOS - if you have Samsung 8E RAM there is a specific BIOS that will probably be best for your card - if you have Hynix RAM there's an optimized BIOS for you as well - etc. etc.
The only real solution is to check out your card's RAM and core, write the numbers down, and start doing some research on the best BIOS you can use. Or you can just try a bunch, and decide based on results
.
Read this thread
HERE for some more info on the Giga-Cube BIOS I'm using, which is optimized for 8E-GCXX RAM chips.