>>16
The speed-up factor can be up to 8x for SSE2, but typically it's about 2x-4x for most tasks. LoseThos would probably benefit a lot from it specially if it does silly things such as software-painting the whole screen 60 times a second.
>>26
SSE was mostly floating point, whereas MMX is integer. SSE2 added integer support and is what effectively replaced MMX. Now we have the same story: AVX is FP only and therefore useless for a lot of stuff, AVX2 is going to add integer support.
Also while we're at it, MMX is 64-bit wide, SSE(2) is 128, and AVX(2) 256.