Hi and welcome back!
I think the "res fix" mod you're referring to is this one:
http://www.subsim.com/radioroom/down...o=file&id=1092
For best results, copy and paste the instructions in the Description at that link into a temporary txt file for quick reference after you download it. I don't think a full version of them is included in the readme file that comes with the download (at least I had to refer back to the mod's page here to work it all out when I first installed it).
IIRC from my own experience I was able to get the game to run in a widescreen res with this mod, but because the graphics in some screens are not formatted for that, it caused certain things to be stretched out (particularly circular objects like the compass) in a way that I didn't like so I use it to run the game the way you describe, in a 4X3 aspect ratio with narrow black bars on each side... just at a higher res than the game's native one.
Two other notes on this mod: when you run it, it will change the way you take screenshots - you will need to use the PrintScreen key on your keyboard instead of whatever way stock/GWX does it. Also, many people have found that trying to take a screenshot when running this mod causes the game to crash and AFAIK no one's yet figured out why or found a solution. It's not a problem I've had but it seems pretty common so if you use it you might want to load up a single mission first and try a screenshot to see what happens rather than have the game crash on you during a critical moment in a career patrol.
Also if you like to fiddle with things in the mission editor, this mod will bork that up because of the resolution changes. The "fix" is to temporarily rename the file d3d9.dll that the mod will have you copy to your game folder. Then the mission editor will work and look fine... just remember to change the file name back before you load the game again!
There are also some widescreen mods out there, where people have reformatted the graphics to fit a WS aspect ratio with no stretching and no loss of available screen space. I think there are two, with threads on both of them in the Mods Workshop forum. Don't know if they are still considered "works in progress" or if final versions have been released. The only catch is that (last I checked) each one was designed for a particular WS res and might not work (or work as well) for anyone using a different res. This may have changed as I have not been following those discussions for a while now - but check it out if you're interested. A search for "widescreen" in that forum should turn up the relevant threads.
One of the most exciting and relatively new mods out there is FM_New_Interiors, which adds significant "playable" area to the inside of your u-boat if you are playing a Type VII, including an officer's mess and everything from the control room back to the diesels. The relevant thread is stickied at the top of the Mods Workshop forum. I tried it out and it was awesome, but I removed it while troubleshooting some other problems... I hope to get it back working again for my next career.
Anyway FWIW here's a list of the mods I'm using right now... others may chime in with their current selections, but what you like in a mod depends on what things you want to add or enhance to enjoy the game more, and that's completely subjective so everybody has a different set of "must haves." If you have any questions about a particular mod give a shout and its devoted users will probably pipe up with info about it and why they like it.
GWX 3 St Naz and Schluese and other units V4
WB's GWX campaign with VonDos' ships v2.7
TMT v2 + ThomsensShips V4.2
WB - TMT fix
WB - add Q ship
GWX - Alternate Loadscreen - Full Circle
GWX3.0 Contact Color
Waterstream+Exhaust Combi V2.3 for GWX3
Waterstream+Exhaust Combi - Single Exhaust for TypeII
WBNN Mission Orders Lite
Rapt0r's Unis V2.0 Grey [All Leather No Medals] Modified
FK's Valkyrie Cap Emblem
SH-5 Water for GWX 3.0 V0.9 Atlantic campaign (default)
Sobers 3D waves
Foam
LifeBoats&Debris_v4
DD_OH_V3.09_with_GWX_Integrated_Orders
DD_OH_V3.09_ConningDeckCam_Fix_20091210230737
Grey Brown Cloth Map
FK's Sound Mod
Scirč-Tikigod- repeated order
OLC's Modified Searchlight Beams for GWX3
GWX - Enhanced Damage Effects
GWX - Captain America's Officer Icons
Pascal_Port_People
b25_SF@Grass_1024_V2
sobers real smoke darker and a brown tint1.1
Flags_enlighten
Torpedo damage Final
electric torpedo trail fix
Turm 7B Your Way
Black Boat - VII - NZ
JSGME (the Generic Mod Enabler) is by far the easiest way to install and uninstall most mods, it will have been included with your GWX install but I recommend downloading the latest version of it.
Also if you don't already use SH3 Commander, I recommend that as well - it will let you tweak many aspects of the game, give you some cool options that you wouldn't otherwise have, and let you manage your crew and career in ways that far surpass what you can do in-game.
Both JSGME and Commander have dedicated threads that are stickied at the top of the Mods Workshop forum.
Happy hunting!