Upgrading a GeForce 7800 GTX

Hi guys. I have a pretty old but not bad computer. My configuration is as follows:
Pentium Dual core 3.00 GHz
Motherboard Asus p5qld pro
Ram 2x2 GB DDR2 Kingston Hyper X(@ 800)
450W PSU
MSI Geforce 7800 GTX(512 MB ) (or GT i don't remember exactly but i don't think it actually matters)
Hard Disk 250 GB WD Sata

I would like to upgrade my graphics card but i m troubled because even though most cards have more memory than my card, they don't always have greater memory bandwidth(mine has approximately 55GB/s). Does that really matter and if yes does that mean i have to swing in 250 $ for a card??? Also could you recommend me a card that could do the job way better that my current one?? My ideal pricerange would be somewhere below 200$.

I m using my computer for mild gaming and designing. For example, last year i played Football Manager 2010 ,Dragon Age Origins and one or two other games like Medieval Total War 2 and that was about it. This year it will probably be the same as i only plan on playing FM2011 ,Dragon Age 2 and Diablo 3(if and when they come out).
My configuration as it is,was more than decent in Dragon Age and even though it did not support full settings, it did support pretty high graphics.
Thank you in advance for your help.