Although it is quite easy to completely replace the hard drive within the X51 (remove the Graphics bracket 2 screws then remove a screw on the drive before sliding it out) we feel that leaving it in is best, then installing a SSD on the spare SATA port, storing it elsewhere with the ideal location being the top of the optical drive caddy. This will require the user to purchase a short SATA cable and splitter for the optical drive power but is otherwise a simple process.
The GPU upgrade is also simple, remove that bracket, flip the clips holding the card in place and remove it from the slot. A new GPU can be placed in though Alienware note it should use no more than 150w. That said even under full load the high spec X51 comes nowhere close to the PSUs 330w so there is more room for upgrades in there, in theory.
We wanted to see how far we could push things so went for the 7950 in this case. That highlighted some considerations users will need to be aware of. Firstly cards of similar size to the reference GTX 555 will drop in easily, that's the likes of a GTX 560, Radeon 6870 and so on. Longer cards are supported but we need to be a little creative. For example the XFX 7950 is a worthwhile candidate as once we remove the heatsink shroud the shape of the cooling fins used offers a better fit than the standard model (though it is VERY tight and some tweaking of fan attachment is required). Additionally we need to place protective tape over the wiring hook on the bracket so that it doesn't short on the back of the GPUs PCB and cards with rear power connectors rather than top are easier to fit as the top edge doesn't have a lot of room... it just allows for top power mounting (2x 6-pin).
So in summary, SSDs fit with wiring a consideration and the 7950 sized cards also fit though heatsink, fans and wiring are an issue. This means that the upcoming 7800 series or mainstream next gen GeForce cards may be a more user friendly option (we will only know at release though).
So what impact would these changes have on performance?
Looking at SSDs first we have the Seagate mechanical drive to use as a baseline and the Samsung SSDs that Alienware are fond of score around 278/270 MB/s, nearly 3x faster. A Samsung 830 Series SATA 3 drive hits 550/410 MB/s though which is a huge gap. That doesn't tell the whole story though as the whole system is more responsive with an SSD, from booting to general use and even opening games and loading levels benefit from the SSD being used.
For 3DMark performance a 7950 combined with i7-2600 scores double that of the GTX 555 when using the performance preset. Real world gaming benefits more though with the Radeon over twice as fast, and even then limited by the game being capped at 112fps. The use of a Radeon 7900 would also allow us to game at 5760x1080 with 3 screen Eyefinity too which is a huge bonus for a compact system such as this.