Nevada Skies is very cool and adds a load of weather effects to the game which improve immersion immensely. Also on their site is install instructions to either reduce the XP rate or make it so you can level past 50, add to skills above 100, and also have a perk every level rather than every other.
There are plug ins for this mod to get from Nevada to DC if you already have a character, look on their forums for Big MT transportalponder mod or repait the vertiberd mod. It combines the 2 games into 1, so you get the gameplay enhancements like Irons sights, DT, additonal ammo types etc. If you have Fallout 3 I'd recommend Tale of Two Wastelands. Just search for them on google or NV nexus and you will find them. The rest are mods I use and are purely my own personal taste, but I do recommend them highly. I have another one called the unified hud projectthat lets you move the HUD around and add extra things like carry weight and current rad count and even a clock. I can't remember which HUD mod does it, ethier Darnified UI or MTU that make the interface less ridiculously big.
4gbNV also helps with performance, especially if you intend to get HD texture packs. ABSOLUTE essentials are New Vegas Script Extender, which most mods need in order to work, and get New Vegas Anti Crash and New Vegas Stutter Remover to massively improve performance.
