Old Desktop GPU vs. new Laptop GPU

[III/JG53_Don]

Hi everyone
Currently I'm playing CoD on my Laptop (i7 2630qm 4x 2,0ghz, 4Gb Ram, Geforce GT540M) with medium to high settings and get reasonable fps as long as no clouds are around

Since my Desktop PC is too weak to run the Sim smoothly (Core 2 Duo 2x 2,4ghz, 3Gb Ram, WinXP) I'm thinking of partially upgrading my PC. I would change my old Core 2 Duo/Mobo and Ram with a new 2500k CPU, Asus P8P67 Mainboard and 8Gb Ram (eventually thinking of buying a better CPU fan as well) with a Win7 64-Bit OS.
Due to the fact that i don't want to spend too much money on it (the upgrade above would cost at about 330-380€ max), the Desktop GPU would stay in this system. Its an old Zotac Geforce 9800Gtx+ with 512 GDDR3.

I did a little research to compare this Desktop Card with the mobile one of my Laptop and found out that 2 comparable Desktop GPUs, which are slightly better respectively worse than the Geforce 9800Gtx+, get in average double the fps of my Laptop Geforce GT540m in games and double of the points in Benchmark programs.

But since the Laptop GPU has 2Gb Ram and my old GPU only 512 I was wondering if this old card would still be a maybe 50% better card than the new GT540m for Cliffs of Dover.
Can anyone give an assumption if CoD will better run on a 2500k, 8Gb Ram, 9800Gtx+ system or on my laptop?
Thanks for your help!!