I believe I am sitting near Nito, in the Tombs. I have killed the Scaless Dragon. This is not my type of game, so yah the curve for learning is above my head. Am I fine with the End and Vit? I don't mind putting a point in Str. Can I make my Silver Straight to +10? And I should give all my embers away? Reason to keep all the boss souls, or use them on something?
Chaos Blade looks cool.
The weapon upgrade system in this game is pretty convoluted. There's weapons that you can take to +5, +10, +15, some weapons are eligible for fire/chaos/lightning/divine/occult paths, some can be enchanted with spells and resins and some cant, various upgrade materials required, etc. That's why a wiki is always useful.
http://darksouls.wikidot.com/silver-knight-straight-sword
I think that's the one you have, right? if so, it's maxed out at +5 (like all of the weapons that require twinkling titanite for upgrading - I think). It has E scaling with STR, C with DEX. So that means a point in DEX should add more to your AR than a point in STR. But since your DEX is already 40, i'd probably leave them alone, because after 40 the difference isn't as noticeable. A point in STR probably won't do much for that weapon. That weapon can also be enchanted with things like gold pine resin or crystal magic weapon, if you're so inclined.
Basically, if you're going to upgrade str/dex, i'd do so based around the requirements/scaling of the NEXT weapon you plan on using, not the silver knight straight sword. You're already almost done with the game. If you already meet the requirements for the next weapon you want to mess with, i'd put any additional SLs you get at this point in VIT and END really.
I haven't used the chaos blade, but I was a fan of queelagg's furysword on my first playthrough. That uses a boss soul, and it's worth making. I don't have much experience with the other boss weapons.