u don't need any wider.. 18mm is plenty wide for 95% of users.
even for me.
as for other accessories, u need to know why u "need" them. i think
the next thing u might wanna invest in is a tripod. don't worry
about ur 2.8 lenses yet, they are really very expensive. u can go
for third party brands of 2.8 lenses (eg, nikon afs vr 70-200/2.8
is $3k+ while the sigma 70-200/2.8 is er... $1.2k? something like
that lar...) but quality differs. it IS noticeable.
if u want a strong but not so ex 70/80-200 i suggest u get a second
hand afd80-200/2.8at's wat i'm usnig) but dun forget.. it's 200mm
not 300. if u wanna go to 300 @ 2.8, er.. then er... gimme the
money i'll buy it for u. 
