help upgradingmy pc into a gaming monster..

hello everyone..nid help badly from you experts..
i just bought myself an hp pavilion slimline s3321p pc.... and i plan to turn this small little pc into a "gaming monster"(preferably good enough to play assassins creed and crysis at max performance)... but the things stopping me from doin just that are these factors:
1.small casing
2.power supply is only 160w
3.only one pci-e slot available
(in which these are stopping me from getting a good video cards with big coolers or heatsinks and require a bit more power)

and if y'all would be so kind... could you write down a list on what is the best suited video card("take into consideration the size of the video card your recommending" ) for my pc and what other parts of the computer should i upgrade to make this a respectable "GAMING COMPUTER"...(nothing too expensive though just as long as it can play "crysis,assassins creed and other high end video games)

here are the specs of my computer:
HP Pavilion Slimline s3321p

Manufacturer: Asus
Motherboard Name: M2N61-AR
HP/Compaq motherboard name: Acacia-GL6E

Athlon 64 X2 (B) 5000+ 2.6 GHz (65W)
GeForce 6150SE nForce 430, Integrated graphics
Memory Installed 2 GB
Maximum allowed 4 GB (2 x 2 GB)
Speed supported PC2-5300 MB/sec
Type 240 pin, DDR2 SDRAM
Power Supply 160 W
TV tuner card
High Definition 6-channel audio
ALC 888S chipset
SPDIF out (coaxial)
PCI...One (TV tuner)
PCI Express x16(currently empty)
Windows Vista Home Premium with Windows Media Center (32-bit)
AMD Live

Firstly i'd get a new case if i were you and transfer all the internals to it. An Antec Sonata 3 would be brilliant for you as it also comes with a decent 500 Watt PSU and looks pretty smart as well! This case/PSU combo will set you back about £70 which is pretty cheap!

This would then allow you to get a decent graphics card, something like an ATI HD4870 would be perfect for your system, they retail for about £170 if you look pretty hard.

Finally if i were you i'd add 1gb more of RAM as sometimes games get bottlenecked by 2gb but i've yet to find a game that gets bottlenecked by 3gb!

oh sori forgot to specify the hard drive also.... the hard drive in my computer is 500 GB SATA 3G (3.0 Gb/sec)...
ok lets say i money isnt a problem then.. what upgrades would you recommend for me??? take in to consideration the size of the parts you recommend for me.... wouldnt want to buy an upgrade that wouldnt fit in my pc...