I am using a dell vostro 1220 laptop with intel dual core T6670 CPU and 2GB ram.

I Know this a very low end PC. But I would like to know the best possible GNS3 topology i could run on this machine.

It is not a matter of topology. It is a matter of number of routers you can use and memory requirements of any router in your topology.
My lab PCs are:
1) old Dell D630, T7250 CPU, 3GB RAM running XP
2) Athlon X2 255, 8GB RAM running Linux Debian

With my Dell I can run 3640/3660/3725 routers with 10-12% cpu usage (it means: 10-12% of each core, as seen on Task Manager) per router. So, in theory I can use up to 8-9 routers.
But... it can go higher, especially when traffic is higher - so running topology with 5-8 (depending on topology and/or features) routers is all my CPU can handle. Your CPU performance is ~25% higher (according to cpubenchmark.net), so you should be able to run up to 8-10 routers. And with 10 routers you can create quite complex topology, at least at the beginning :)
My other PC (running Linux) is able to run labs with 15 routers, but CPU usage is ~85% on both two cores.

Some useful hints:
1. Try to find really good idle-pc value. With old core2duo CPUs every percent of CPU usage counts, really
2. use IOS 12.3 (and maybe last 12.2 images too) when possible - they have lower memory requirements, and low number of features, so they SHOULD consume less CPU power.
3. if you plan to simulate VPN topologies, do it with pure GRE tunnels and use IPSec as absolute last resort - IPSec has huge impact on CPU usage when carrying traffic

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If you can see any inaccuracies or other language-related mistakes in my post(s), please forgive me - English is not my native language