i agree with roger: if you want a nice transition kit with a big planning potential and which will llow you to progress on go for the SB GO 139 or for the 144(?? not sure about that last volume). With your weight eighter should be perfect to just float on in low wind and in higher winds these boards can be (with a slight fin adjustment sometimes, at least when your trying to sail over 20 knots with it or in heavy chop) cntroled perfectly. As for sailsizes the GO's accomodate a large range. Personally i use it with 6.2 (if the wind is gusty and i can't sail my 115 L board) and bigger (i'm getting an 8.8 from a friend) but i have used it with a 5.5 in serious chop with a 40 cm freeride fin which worked very well .
It sounds to me that you're sailing a lake with not too much strong winds so you might want to take a GO (or a futura if you think it's better for you ... i haven't tested a futura and i've been using my go for 3 seasons now or so? and i won't replace it untill i've wrecked it) that can accomodate sails up to 10.5 mē or s. this way, when you get better and more accustomed to larger sails you can take out your board on low wind days and still plane with it (off course you're gonne have to save up for a big rig
but those are problems for later).
hope this helps,
(hurray post 200!)