Sooner535 :
ur6beersaway :
Which HPE model # & we can max it out for you.
HPE-337C
Since you have a 300w PSU the best GPU to buy is either the Radeon HD 7750 (73w max power) or the GTX 650 (68w max power). The GTX 650 is almost 10% on average more powerful than a stock speed HD 7750 and 2% faster than an OC'ed HD 7750, but it also cost more. Here's a performance chart and a link. Click the link for the source and review with benchmarks:
http://www.techpowerup.com/reviews/HIS/HD_7750_IceQ_X/28.html
Based on newegg prices the cheapest GTX 650 is the following MSI N650-MD1GD5/OC GeForce GTX 650 1GB for $99.90 after a $10 rebate. It is slightly overclocked by 71MHz.
http://www.newegg.com/Product/Product.aspx?Item=N82E16814127703
The least expensive Radeon HD 7750 with DDR5 RAM (don't go with DDR3, there is a performance penalty) is the PowerColor AX7750 1GBD5-DH Radeon HD 7750 1GB for $70 after rebate.
http://www.newegg.com/Product/Product.aspx?Item=N82E16814131461
However, I do not like the design because the heat does not exhausted out of the back of the graphics card. It simply gets dumped inside the PC case. Therefore, I rather buy the following $75 after rebate XFX FX775AZNJ4 Radeon HD 7750 1GB DDR5.
http://www.newegg.com/Product/Product.aspx?Item=N82E16814150663