I've seen both こんばんわ and こんばんは used; which is correct here? If we interpret the は as the topic particle, は would seem correct, but it seems that わ is used quite frequently anyway...
こんばんは is correct, according to that page in Japanese.
My gut feeling is the same - 今晩は -> こんばんは.
That said, a cursory Google of こんばんは yielded 13M hits, whereas こんばんわ yielded 26M.
こんばんは (今晩は) is the one in the dictionary for "Good evening!". I think that こんばんわ has a cuter feeling, maybe a little softer. It's a total guess, but it might be related to the feminine わ at the end of sentences. Actually, this page seems to be saying it's related to 和 (わ - peace) which gives it a nicer feeling.
The right way is always こんいちは, こんばんは with the particle "HA, は" It's like that because it's the old Japanese way to say "good day". the "ha / は" works the same as "is": "Anata ha, watashi ha" 「あなたは」, 「わたしは」 meaning something like, "today is a good day" etc. After some time, it becomes fused in everyday colloquial language. Also, we should not forgot that Japanese Kana only represents "sounds of spelling". We are allowed to write こんばんわ with "wa", but we need to check where we are using it. This way of using "wa" can only appear in informal text such as web chats, with close friends, and manga. For any document that is official or with strangers it's mandatory to use the formal way, with "ha" and preferentially using kanji with the particle 今日は, 今晩は.