after lookin at your specs. I'm guessin you have a HP computer.. that came with 1GB of ram. you added 2 for a total of 3GB of ram. and you have the integrated 6150 SE GFX.
I had one. an HP with a AMD x2 5400+ 2.8 GHz with 1GB, I added 2. and Mine only had a 250w PSU. and I added a 8600GT and it worked perfectly fine. it played COD4, BIOSHOCK, and Crysis. Crysis struggled, but worked. you having only a 19 inch monitor you'd be more than fine.
There's no program that will tell you what GFX card to get. but you can get Power supply program that tells you how big of one you will need with what all you use. but you're fine. You can't get a new ATI 4850 or 4870, or a New Nvidia GTX260/280 without upgrading your PSU. but if you don't need something extreme like that. Then you'd be fine with a lower video card.
I'd recommend getting the Nvidia 9600 GT. not gs, only GT.
you could get an Nvidia 8800GT, but I think you might need a bigger PSU for that also. If I was you i'd just get a 9600GT, but if your planning on doing any new games, you need a new psu and a 8800GT, or something better.