Sorry I don’t know of any sewing forum sites. I sew a little bit – mainly quilting, blankets and a few simple curtains…
I think a top stich can be functional and/or decorative, depending on what you using it for. Basically it is just a stitch you are putting on the top of the fabric – maybe it is holding fabric together (from the bottom side) and maybe it is just a pretty stitch to add some decoration and maybe it does both.
In this case, I sounds like you are joining the 2 fabrics together first, so I would asssume that you then are ironing the seam down/open/flat and stitching from the top-side about a 1/4" from the existing join/seam. I think the purpose might be more funtional and would be to hold the fabric join flat as well as provide some added support. If you use nylon/invisible thread then you wouldn’t really notice the added stitches – I’ve not used nylon thread but I think it’s a little expesive. Depending on your curtain fabric, you could probably just use a matching thread and no-one would notice. I’d also think just a regular straight stitch is what you would use in this case, unless of course you are trying to add some decorative stitching – in which case you would probably use a pretty contrasting thread and a fancy stitch.
I don’t know that the topstich on the curtains is really needed, but I guess that might depend on how long/heavy they are and maybe if you are going to be opening and closing them alot. Also on if the seam you are joining is vertical or horizontal. I’ve made a couple of simple curtains around the house and not done a top seam on any of them. I often head down to Linen and Things or JcPenny’s or wherever and look at curtains similiar to the ones I am making and inspect how they’ve made them.