You're actually not as bad off as you think. That gtx645 is something custom for dell and will out perform a gtx650 and overclocks very very well. So just doing that even should get you beyond console territory at least for now.
Memory is fine along with the cpu.
The machine is provided power by a standard Dell ac adapter which you should have received a 240w one with the x51 and they can go up to 330w. You can find them on ebay pretty easily if you need more juice.
Inside the case you will find almost enough room for any gpu you could want. The system also has two pcie6 adapters. People have crammed gtx680s in them and such. Just be careful with the measurements or be prepared to modify the system. The only thing I really wouldn't suggest putting in there are 7950,7970,290,290x and their derived gpus. Most gtx760s and r9-270's fit in it just fine or if you're looking for a pretty good punch on the cheap a gtx750ti is the absolute lowest I would go seeing as like I said with minimal ocing that gtx645 of yours goes a long long way.
Also the machine inside has three sata ports to use. You can fit an ssd over the optical drive generally or pull the 3.5 drive and use two 2.5 drives. There are other configurations you can do as well depending on how adventurous you are.
Edit - Side note for the gpus. The case is designed for a "blower" style gpu. If you get a gpu that forces air into the front of the case. Reverse the gpu intake fan in the front of the machine or the gpu will force a ton of hot air into the rest of the machine and feed that hot air back into itself too.
If you are unhappy with how the cpu fan sounds there are several options for that as well.
Used to work for dell and also bought five of these things on sale for family so ive spent more than my fair share of time with them.
Edit 2 - If you paid more than 450-500 for that thing look at delloutlet.com next time. Same crap but stuff people returned. Just stay away from inspiron , Vostro, and the lower end XPS stuff. While building your own is fun and what not sometimes OEM vendors offer stuff you cant make yourself or in my case the next business day world wide warranty is something im happy to pay a little bit extra for.