That 250w PSU is seriously going to limit your upgrade potential. Best I can suggest for semi-decent performance would be something like a GT 240 at the most. Even that would be touch and go. Maybe look for an 8600 or 9600.
Alienware M11x R1 Netbook:
1.3GHz Core2 Duo U7300 (OC'd to 1.7GHz) | 8GB Corsair DDR3 1333MHz | nVIDIA GeForce GT 335M 1GB/Intel GMA 4500MHD Hyrbid Graphics System | Western Digital Scorpio Black 500GB | Microsoft Windows 7 Home Premium 64-bit
Microsoft Certified Desktop Support Technician | CompTIA A+ Certified