please dont get a cheap board you will be disappointed. You really want a board with good chipset coolers on them and a high flow case but it depends what you plan on doing.
sli/crossfire?
here is the main difference
990FX: AMD's enthusiast line is, as displayed in the table, primarily advantageous for its added PCI-e lanes and bulked-up multi-GPU support, allowing up to four GPUs (x16/x16, x8/x8/x8/x8). Additionally, the 990FX's TDP is calculated at 19.6W, certainly making it undesirable for low-spec systems, like HTPCs. The 990FX uses the SB950 southbridge and has a total of 42PCI-e 2.x lanes.
990X: In the performance section, which is probably what most of our regular system builders should consider, the 990X is equipped to handle x8/x8 graphics arrays, operates at 14W TDP. The 990X also uses the SB950 southbridge and has a total of 26PCI-e 2.x lanes.
970: This is certainly the 'mainstream' chip. With no support for Crossfire/SLI and only 26PCI-e 2.x lanes, the 970 is best-known for its low-TDP (13.6W) and cheap buying options. The 970 uses SB950 and SB920.