Well to me (as I own the 965) the processor works very well. I dont know what games you play but I would suggest at least a 7770 it will kill your 6450 and can be found for around $100 after mir. If you can stretch I would go with a 7850 which can be had for 159(at the cheapest after mir) also comes with bioshock and tomb raider if that's important. Not trying to push your budget to far but I would probably try to get a 2 gb version if you game at 1080p.
I buy amd products so someone else might have to chip in if nividia has better cards in the price range but yea.
Some cards to consider
Sapphire vapor-x 7770 Sapphire makes good cards and the vapor-x according to many is no exception. Sapphire reference HIS Icooler
Ive had pretty good experience with my HIS products XFX Reference HIS
At 159 (after MIR) and 164 respectively these are the cheapest 7850's not worth mention other cards because itd be more worth it to get a cheap 2 gb version. Gigabyte 2GB 7850
194 and 179 with MIR absolute cheapest, there are a few others that come to 179 with MIR as well.
Alright hopefully that helps I know you said 140, but in my defense you said about 140. So I gave you a few cards within a 40 dollar direction either way albeit after MIR. IMHO stretching for the 2gb 7850 would really be worth it and you'll be able to play the bioshock and tomb raider games it comes with quite nicely. ALso imo stay away from the 2gb 7770 they are bandwitdth choked and aproach the 7850 price range.Hope this helps!
The 6450 is essentially designed for HTPC (ie, playing back HD video) and gaming at fairly low resolutions 1280x720 or less on low to medium settings. It's only good if you play older RTS's and even older FPSs.
At your budget level, look at the GTX 650 Ti. If you can find them, last generation GPUs give you better performance for the price in exchange for higher power consumption. Look at the HD 6870 and GTX 560 Ti.
The 6870 is also a very good choice, Also the 560 ti is a pretty good car as well (AFAIK im not an amd fanboy ive just only bought amd cause at the time it was more budget efficient) The 6870 cannot be bought retail for a fair price, so off to ebay. I cant really help you there I buy a lot off ebay but I am not a huge fan off bidding wars but it looks like you can find something well under your budget that would outperform the 7770 if you have bidding skills and dont mind "slightly" used (which to me who knows what that means).