The 9500GT will give you a decent boost in performance, but the question is how much is it selling for? Also what resolution do you play games at? To give you an idea about general price / performance (lowest prices I can find on Newegg):
GeForce GT 220 - A little faster than the older 9500 GT. Performance is between the 9500 GT and 9600 GT. Not bad for $45 (after $15 rebate) + $3 shipping.
http://www.newegg.com/Product/Product.aspx?Item=N82E16814150493&cm_re=gt_220-_-14-150-493-_-Product
Radeon HD 4670 - Definitely faster than the GT 220. This card is overall a tiny bit faster than the newer HD 5570, and it lacks DX11. Then again the HD 5570 is rather weak for a DX11 card. Price is $50 (after $10 rebate) + $4 shipping. Only a little bit more than the GT 220 and definitely worth the small price difference.
http://www.newegg.com/Product/Product.aspx?Item=N82E16814150450&cm_re=hd_4670-_-14-150-450-_-Product
Radeon HD 5570 - Overall, slightly slower than the older HD 4670. It uses a little less power (30w vs 45w) than the HD 4670. While it is a DX11 card, DX11 features lower the performance of this card. Price is $55 (after $15 rebate) + free shipping. I would get the older HD 4670.
http://www.newegg.com/Product/Product.aspx?Item=N82E16814102875&cm_re=hd_5570-_-14-102-875-_-Product
GeForce GT 240 - Better performance than the previously mentioned cards. It is slightly faster than the older 9600GT. Price is $55 (after $20 rebate) + Free shipping.
THE REBATE EXPIRES TODAY - 1/20/2011. Definitely the best card so far in the $50 - $60 range.
http://www.newegg.com/Product/Product.aspx?Item=N82E16814125304&cm_re=gt_240-_-14-125-304-_-Product
Radeon HD 5670 - This is a mainstream DX11 card. It has good performance for an inexpensive card. Also there is no performance difference between 512MB of VRAM and 1GB of VRAM, so might as well go for the cheaper model. Price is $65 (after $15 rebate) + $2 shipping. It's $10 more than the GT 240, but I think the jump in performance is worth that $10.
http://www.newegg.com/Product/Product.aspx?Item=N82E16814102871&cm_re=hd_5670-_-14-102-871-_-Product
Radeon HD 5770 - Of course this is the most powerful card in this line up, but it is another $33 more than the HD 5670. Priced at $100 (after $20) rebate + free shipping.
http://www.newegg.com/Product/Product.aspx?Item=N82E16814161338&cm_re=hd_5770-_-14-161-338-_-Product
Below are some benchmarks of all the card at 1680 x 1050 resolution. There are also other resolutions a well in the review. Be aware that all of the card mentioned above have rebates involved so if you are not going to buy them soon, then prices will change. But you can always wait for new rebates. I think the GT 240 is the best card if you are really on a tight budget. The HD 5670 is not that much more and if you can scrape up the extra $12, then that's the card to get. I assume the HD 5770 costs more than what you currently have, but this card should last you a while. So spending more now, can save you money in the future.
Make a deal with your parents to loan you the extra money for the HD 5670 or HD 5770. I don't know how old you are, but maybe you can get an advance on your allowance (if they give you one) or tell them you will help out with some extra chores. I think offering to do extra chores is the way to go since you are not simply begging for more money.
Source of benchmarks:
http://www.techpowerup.com/reviews/Powercolor/HD_5570/1.html