For baseball, MVP 2005 w/ conversion and add-ons. You probably won't find a copy for under $40, but that's better than paying what you would for the craptastic 2k game. Use
www.mlbmods.com for the main 08 Total Conversion. Then look for other add-ons that you want. PM me if you want more info.
For hockey, NHL 2004 with the amazing modifications they've done as well. Check out Randy's site for full information:
http://z6.invisionfree.com/flyermania/index.php?act=idx
For football, though I'm not a big Madden fan..... I'd say the last Madden they made for the PC ... Madden 08.
Can't speak to basketball, but when I asked earlier a lot of guys said NBA Live 05 for a lower end PC. They have some great mods for that as well. Don't know how the 2K games play on the PC.