I'm considering buying the XP boost to help me get to level 20 faster so I can get the best runes. Before I go spending my RP, about how much of an XP boost does it give per game?

Here's a chart with how much you get. Scroll down a bit to see it.
http://leagueoflegends.wikia.com/wiki/Xp_boost

I would avoid xp boosts as it just causes you to level faster but not gain pvp experience as fast.

Agreed. Players should take advantage of the learning curve they can achieve by leveling up. Hitting 30 too soon or leveling to 30 exclusively in AI matches can set players back when they do break into level 30 PvP. I know players are eager to get tier 3 runes, but also keep in mind that leveling faster than you can earn IP won't help your rune issues anymore than getting to level 20 in the first place.

I recommend IP boosts while leveling, stock up as much IP as you can those T3 runes are crazy expensive. Don't be shocked when you spend 400-800 on basic runes and 2000 on Quintessences.

I have played quite a lot of games with premade level 30s on my team so also many 30s on their team and my account got banned so i played another smurf till 20 all on pvp, but im still level 25. Should i try to get an xp boost and get to 30 for better runes etc? And also to play ranked? Although im not sure if i will be ready for ranked.

You don't get any extra special runes at 30, so no. Tier 3 is unlocked at 20.

Also, if you are unsure you are ready for ranked, start by playing Normal Draft. Once you understand the "meta", you can play more than just 2 roles effectively, you have at least one extra rune page, and you can finish 90% of your games even/positive you might just be ready.

Months old thread...