Need advice on upgrading my computer.

Hi everyone, I'm Lance and I'm new here, nice to meet you all.

So here's my current specs:

E5700 Dual core 3.0ghz processor
ATI Radeon HD 5450
4 gigs of ram
Windows 7 64-bit

It pretty much sucks, I know, and I can't argue about that espicially after playing The Witcher 2.. Heh, that was torture. Anyway, first of all I'd like to point out that hardware is damn expensive where I live, like if a graphics card costs 100$ in USA it would cost 600 of my currency (trust me, you don't need to know where I live, just know it's expensive  ) So, after playing some recent games and most recently The Witcher 2, I've decided I can't stay with this rig much longer. First I obviously needed a new graphics card since everywhere I just keep hearing HD 5450 isn't made for gaming, so I think about going for a HD 5770, I hear it's decent and I think I can afford it. Then I searched around for a bit, and heard some people say that the E5700 would bottleneck the HD 5770 (please include it in your answer if it would) so I'm thinking about upgrading to a quad core, I heard about the Intel Core 2 Quad Q6600, I know it's kinda old but I heard it's decent, plus the older it is the cheaper so...
I need your advice guys, will upgrading from a HD 5450 to a HD 5770 and from an Intel E5700 dual core to an Intel Core 2 Quad Q6600 (if it's necessary) make a big difference? will the E5700 bottleneck the HD 5770? And Last but not least, how much would a HD 5770 and Intel Core 2 Quad Q6600 cost in dollars nowadays? I need to make my calculations.

Important note: I will be gaming at 1440x900 and not any higher, so tell me how good would the HD 5770 and the quad core be at playing recent games (Say, The Witcher 2) at 1440x900 and a mix of high/meduim settings (all medium if necessary).. I really hope I get positive answers because I can't afford any better upgrade than this currently   Thanks for reading and thanks in advance for your answers.. Any help much appreciated.