This should be been posted in the CPU section since you would get more responses, but I'll help you out. The good new is your mobo can use any of the C2Q quad core CPUs as long as you have the correct BIOS version.
I'm just going with Newegg since I like their search engine a lot and I've been buying from them since they first opened shop back in 2001... wow it's been 10 years. There are 4 C2Q CPUs available for sale from $240 to $350.
The best bang for the buck would be the Q9550 for $300. The price is relatively high compared to other more current quad core CPUs that are more powerful. But due to low supply and low demand, the price is high.
The i5 is 3.1GHz (3.4GHz Turbo Boost) vs the Q9550's 2.83GHz. Therefore, it is a faster processor. Not only is it faster, but the Sandy Bridge family of i5 CPUs is around 10% - 15% better performing and the older Clarkdale family of i5 CPUs. Clarkdales performs about 15% better than the Yorkfield CPUs (like the Q9xxx series).
Basically what this means is if the you compared an i5 and a Q9xxx CPU at the same clockspeed, the i5 CPU can process more instructions per clock cycle and thus, more powerful than the Q9xxx CPU.
Turbo Boost is the ability for the CPU to "overclock" itself under high demand situations.
The only issue with the i5-2400 and the Asrock motherboard is the limited overclocking potential. However, since you are using an Intel motherboard, I am sure you are not overclocking your current PC anyway.
I honestly wouldnt bother upgrading it, i would either overclock it (those cpu's overclock really well, should get it to 3.8ghz which would be plenty for any game), or totally switch platform. For the money it will cost you to get a 3ghz core 2 quad, you could just upgrade CPU and motherboard to something newer and better.
thanks to all of you for helping me i have overclocked C2d e7500 @ 3.8 ghz..
and XFX ati hd 5830 @ 850/1050
for this i have changed intel dg41rq mobo to asus P5G41t-M LX mobo , and DDR2 2x2GB 800mhz to corsair XMS3 1x4gb DDR3 1600 Mhz and also installed hyper 212+ system cooler.... and now i'm saving for future...