I have both the Canon 30D and 5D MarkII, and both are very good. There is no autofocus problem. For tracking action, I always use the center focus point and AI servo, works very well. The 5D MarkII is an excellent camera but it is not a fast action camera (only 3.9 fps). It is okay for my kid's soccer, baseball and casual sports photos. The Canon 7D or 1D Mark IV are better for pro sports (or Nikon D3/D700 if you are a Nikon user).
The high resolution of 5D MarkII is great for landscape, portraits and studio photos that has a lot of details, so that you can see these fine details at large enlargements for wall mounting display.
I agree with the above that building a good lens collection is important if you are getting more serious about photography, more important than getting a better camera body. The full frame cameras will do better with the higher quality lenses (that will double or triple the cost of the camera body). I have the EF-S 17-55mm f/2.8 IS for the 30D, which is a very sharp and high quality zoom lens (but this lens is only for APS-C size cameras, so not a good choice if you plan to upgrade to full frame). I also have the EF 70-200mm f/2.8L IS which is a great mid range tele, very sharp, very versatile (sports, portraits, wedding, events, etc) and I highly recommend the IS version (you will need it for handheld shots). I bought the EF 24-105mm f/4L IS as a kit lens with the 5D Mark II, turns out to be a very good lens at a good price, reasonably sharp, very versatile and a good bargain when bought with the 5D MkII. The 50mm f/1.4 is a cheap and sharp lens when stopped down to 2.0 or narrower.
Here are a list of other popular high quality Canon zoom/prime lenses:
EF 16-35mm f/2.8L (landscape)
EF 24-70mm f/2.8L (events, wedding)
EF 100-400mm f 4.5-5.6 L IS (sports, safari/wildlife)
EF 14mm f/2.8L (landscape ultrawide angle)
EF 24mm f/1.4 L II (landscape)
EF 85mm f/1.2 L II (portrait)
EF 135mm f/2.0 L (a very very sharp macro lens)
TS-E 17mm f/4 L Tilt-Shift Lens (architecture, landscape)
But your $3000 budget is not quite enough for the Nikon D300 or 5D Mark II systems. This is because you did not budget for the lenses and accessories.
Here are a list of useful accessories:
A polarizer filter
A neutral density filter
At least one or more external bounce flash that can tilt and swivel
one or more flash diffusers
Photo editing software (eg. Photoshop)
A camera bag with adequate padding to protect your gears
A remote release cable or remote control to trigger the shutter
A lens cleaning kit
If you print your own photos, then consider a Monitor Calibrator (eg. Spyder 3 or Huey)
A grey card or Expodisc (or just shoot RAW and process it yourself)
2 Umbrellas, and softboxes
Studio strobes (I like the Alien Bees, they are high quality and inexpensive), also consider their Cyber Commander which is a great remote control with built-in flash meter
Other useful accessories include gels and reflectors, beauty dish and grids for the studio strobes, color filters
It is also fun to do chromakey shots with digital backdrops (but you will need some chromakey softwares and studio lights)
These accessories are fun and add creativity, expanding the range of photography that you do. They also help you to learn different aspects of photography, to sharpen and polish your skills. They are also adequate for a small home studio setup. My 30D/5D MarkII setup costs more than $10,000. So if your budget is $3000, then consider an entry level D-SLR, some better lens and accessories.