I would concur with Gdad, ukcatfan, YECKIM and others - go for the glass. Even the thrifty fifty (50/1.8) at ~$100 will open up all kinds of possibilities in portraiture. You'll be amazed at what those extra stops at 50mm will get you. You'll be able to isolate your subject, get nice creamy backgrounds, shoot in much lower light, etc. I have this lens and its bang-for-the-buck is incredible. Getting a flash that you can bounce will help a great deal as well.
Regarding landscape, the kit lens is actually fairly well regarded for its sharpness at higher f/stops. You'll probably want to shoot at f/9 minimum in order to get great depth of field, so sharpness won't be an issue. You may find that you'll begin getting into shutter speeds that require a tripod.
I too have a D50 and have shot it enough to know that 6mp is plenty of resolution, particularly when considering how low its noise is at higher ISOs. If you like "available light" shooting, this lower noise is more important than megapixels. I too don't consider moving to a D40/40x/60 a step up, but a lateral move at best. If you didn't have a D50 this would be a different story, and they would be geat cameras to get into the Nikon system with. If you fill out your frame in your D50 and don't need to crop, you can get 8X10 shots at 300 dpi printing resolution, and in practice get great 16X20's when viewed at normal distances (versus pixel peeping).
The next real step up would be the D80, and quite honestly if I was stepping up from the D50 I'd REALLY want to go to the D300!!