AC1 was seriously nothing compared to ACII. Its amazingly good and even people I know who didn't really enjoy the first loved it. One of the best sequels ever.
I'd also highly recommend Mass Effect.
Though if you're looking for something fun that doesn't require a lot of commitment I'd go with Arkham Asylum. Not exactly new anymore but a ton of fun. Uncharted is the most fun ever but its PS3 only unfortunately.
I would actually go as far to say that AC II had a better story mode than Uncharted 2. This is coming from someone who didn't play the first. But the value in the multiplayer of Uncharted 2 is better, and there's nothing to do in AC II after you finish the story, as there's no online or extra difficulty modes, and I've heard the DLC's weren't the best.
Batman was also brilliant, purely because of the fact that it was a licensed game that was completely awesome. GOTY edition coming out soon as well (but only comes with free DLC and 2 payed ones). But it was irritatingly short, and the Challenge Rooms IMO got old fast.
So considering the fact that you won't be getting many more games any, time soon, I would say rent AC II and Arkham Asylum, or better, get each for a week at EB Games, then return them for a full refund. Then, get something like BC 2 or ME 2.
Once you get Bday money... You should seriously consider Red Dead Redemption, bound to be a classic Western game, with tonnes of longevity even in the story.
I'm guessing you have PS2 and 360? I wouldn't get a PS2 game tbh, and I'm amazed that GoW 2 would cost $25 pre-owned, that's surely atleast $10 a bit high.
And don't get The Godfather II, it was terrible, the first was OK, but in any case, wait for Mafia II.
Lots to choose from really.
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