OK, thanks for the tips guys.
I'm on level 4-1 so far I think, and I've not sold one single piece of treasure yet, and I've enough for many tens of thousands worth of credits. I've upgraded my guns as much as I could, just by the gold I've collected, but being my first Resident Evil game (since the PS1) I wasn't sure whether to keep the treasures or not. I was keeping them all this time, in case I got some kind of reward at the end, and had planned to play it through again, and play it differently.
Now I know I can sell them all, and get all this moolah, I'm gunna go mad upgrading some guns next time I play it.
I have been upgrading the standard pistol, and I find it quite effective, it's got me to 4-1 anyway, that and the shotgun and rifle. I got the magnum during level 3, but never found any ammo for it, so when it ran dry, it was useless, and I died, and then changed back to the original pistol again.
So, next game I shall have a play about, and upgrade the pistol, rifle and shotgun.
I'll be playing it through a second time anyway, I have to say I'm really enjoying it, and having a hard time putting it down.
Vin.