It's actually no prob at all dude, very easy to locate and do if I can.
I could even setup diff voltages then what anyone else could, like half steps.
Like if you're looking to up to say 1.45v I could do that, or 1.5v, whatever is actually possible though.
The reason why nibitor freaks is likely do to the device id's on your card's bios.
Unless you have a corrupt bios that is.
Nibitor does'nt have some of the id's.
Also the board id is wrong in it most of the time, it gets that offset right one one bios that I've seen, I think it was a quadrofx540.
Anyways you should beable to open it up anyways, unless liek I said it's bad.
It'll say unknown device or somehting liek that, just pick your core type, I think it had 3.
Gf-fx, gf6 and gf7's.
From there change the voltage from say 1.4v to 1.5v.
You should have to extend the voltage table, if you do it changes nothing, it only gives you more options

.
Ok looking at a gainward 7800gtx bios right now for the heck of it.
Bios ver: 5.70.02.11.24
I just scrolled down, did'nt take me long at all lol, I'm used to the bios

, to find this string:
KDASHARE
Right afterwards is a series of FF'hs.
Those are the 4 tables, 2d, then 3d, then 3d idle, then some unknown one.
Lemme see if I can figuer out the default voltage for it.
Hmm the 2d voltage is lower then I've seen before...
78h.
This if from my experience so far:
82: 1.3v VCore
8C: 1.4v VCore
91: 1.45v VCore
96: 1.5v VCore
9B: 1.55v VCore?
A0: 1.6v VCore?
It has 8Ch for all the rest of the perfromance level's except for the last, which is 00h.
It's a null like performance lv anyways, no one uses it, I wold have to flash it enabled and check it out using the driver cpl just to see if it's even used at all for anything...
Anyways, default core votlage for everythign but 2d is 1.4v.
I "think", guessing, 2d is 1.2v.
I've found that I can clock well o 2d even if I don't mod the voltage for that on my 6600gt, 550 with stock voltage for 2d...
I can't mod the 3d voltage on my card because it does more harm then good.
I can't handle the heat even though I can cool the card somewhat decently with air coolingl.
Anyways

, as you can see it's not an issue at all.
I could even piont it out and have you doit lol.
Anyways try to get nibitor working.
If you can't, attach the bios you should of backed up from your card to here, it should be pretty small, 64,512 bytes if it's valid.
I can at least look at the begining of it to see there's anythign wrong with it, and try getting it to work with nibitor.
It's really not that big of a deal, very easy stuff

.
Edit:
Btw, what do you mean by softmod?
I know there's a way to read back the vid, but that does'nt give you the voltage actyually used unless you know what the vid stands for.
That's only to read back the voltage.
It's actually useless anyways...
Please explain, I'm curious

.