Pvz gw2 keeps studdering

Problem is if the computer has a GT610 now, it's unlikely a beefy medium-end GPU could be placed in without upgrading the Power Supply.  It depends.  There may also be no room for a full-sized card since most GT610's I've seen are half-height cards.  If your PC is really slim and can only fit half-height cards there is very few options but there are options (albeit probably not the best for AAA gaming overall).  If your PC is a full tower (can fit full cards) then check your Power Supply to see what it can support.

http://slickdeals.net/

That website I watch almost every day.  They have a good amount of PC parts listed everyday.  With patience you can get some real deals.

A GTX960 runs about $180 retail and on sale probably can get it down to $150.  That's probably the horse power I'd be looking for though to satisfy Performance and Budget.  Can likely drop down another step and still play this game pretty well but the further down the ladder you go the quicker you'll run into this problem again for future games.

Here are charts on how they stack up to competition and older cards.

http://www.tomshardware.com/reviews/gaming-graphics-card-review,3107-7.html

Buying a used card is an option too.  Older beefier cards will likely require even more juice however and I'm betting your Power Supply isn't going to handle much without replacing it.  It may not even have power connectors higher end GPUs require.