When you overclock too much without adequate vcore on ABIT it usually will give you the BIOS checksum error. There's actually nothing wrong with the BIOS neither that you need a newer BIOS.
If you want higher than 1.85 vcore than you have to mod your mobo.
The BIOS rev is written on the last line next to the date when the comp just start and counting the ram (you know, that black screen with white text and all). It usually only a code like ww, wz, wk or something else. You can determine the bios rev by looking at the date also.
Read your manual! There's a section there that shows you how to find the bios rev.
Like Wild_Andy said, 935 for a 600 Duron is very good. Only few lucky one can be pushed to 1GHz. My 600 Duron can only go upto 850 with 1.85vcore and need 2.4 v to get stable 1GHz.